STI Testing Beyond Syphilis: HIV Algorithms, GC/CT NAAT, and the Herpes Serology Dilemma

 

STI Testing Beyond Syphilis: HIV Algorithms, GC/CT NAAT, and the Herpes Serology Dilemma

The testing rules that have changed, the test you probably shouldn't be ordering, and the window periods that still trip everyone up.

The syphilis post covered one STI in depth. But the full STI panel has its own set of testing nuances, window periods, and pitfalls that new NPs need to master. HIV testing has evolved dramatically with 4th-generation assays, gonorrhea/chlamydia NAAT is site-specific, and herpes serology is the test that causes more unnecessary distress than almost any lab in primary care.

HIV Testing: The Modern Algorithm

4th-Generation Ag/Ab Combo Test (The Standard)

The current recommended initial screen is the 4th-generation HIV-1/2 antigen/antibody combination immunoassay. It detects both HIV antibodies AND p24 antigen, shortening the window period to approximately 2–4 weeks after exposure (compared to 3–12 weeks for older antibody-only tests).

The CDC Recommended Algorithm

1. 4th-gen Ag/Ab combo test (screen). If nonreactive → negative. Done.
2. If reactive → HIV-1/HIV-2 antibody differentiation immunoassay (confirmatory).
3. If differentiation assay is indeterminate or negative → HIV-1 RNA (NAT) to detect acute infection (p24 antigen-positive but antibody-negative = acute HIV).

Key Pitfalls

• Western blot is obsolete for HIV confirmation. The differentiation immunoassay has replaced it. If your lab is still using Western blot, they're outdated.
• The window period still exists: 4th-gen tests can miss very early acute infection (<2 weeks). If exposure was very recent and clinical suspicion is high (acute retroviral syndrome), order an HIV-1 RNA (viral load) directly.
• PrEP patients: HIV testing every 3 months per guidelines. Always use the 4th-gen combo, not a rapid antibody-only test, which has a longer window period.
• Universal screening: CDC recommends all adults 13–64 be tested at least once, regardless of risk factors. All pregnant patients should be tested at the first prenatal visit.

Gonorrhea and Chlamydia: NAAT Is King

Nucleic Acid Amplification Testing (NAAT)

NAAT is the gold standard for GC/CT detection—far more sensitive than culture. Key points:

• Site-specific testing: Urogenital NAAT does NOT detect pharyngeal or rectal infections. If the patient has risk factors for extragenital exposure, you must specifically order pharyngeal and rectal NAATs. This is routinely missed.
• Specimen type: First-void urine (for urethral/cervical) or swab (vaginal self-swab is FDA-approved and equally accurate). Urine testing in women is less sensitive than vaginal swab.
• Test of cure (TOC): For GC, repeat NAAT at 14 days post-treatment (not sooner—dead organisms can cause false-positive NAAT up to 14 days). For CT, TOC is recommended for pregnant patients and when adherence is uncertain; retest at 4 weeks.
• Reinfection screen: Retest ALL patients treated for GC or CT at 3 months (high reinfection rates). This is the most commonly skipped follow-up in STI management.

Pitfall

A negative urine GC/CT in a patient with pharyngeal symptoms or rectal complaints means nothing—you tested the wrong site. Always ask about sexual practices to determine which sites need testing. MSM should routinely be screened at all three sites (urogenital, pharyngeal, rectal).

Herpes Serology: The Test You Probably Shouldn't Order

The CDC Does Not Recommend Routine HSV Screening

This is the most controversial testing decision in STI medicine. The CDC explicitly recommends against routine serologic screening for HSV in asymptomatic individuals. Here's why:

• HSV-1 seroprevalence is ~50–70% in the US adult population. A positive HSV-1 IgG tells you almost nothing clinically useful—it could be oral herpes from childhood, genital HSV-1, or completely asymptomatic carriage.
• HSV-2 IgG has a significant false-positive rate at low index values (1.1–3.5). The test has poor positive predictive value in low-prevalence populations, causing unnecessary psychological harm, relationship disruption, and anxiety.
• A positive HSV IgG doesn't tell you the site of infection, when it was acquired, or whether the patient is or will be symptomatic.

When Herpes Testing IS Appropriate

• Active genital lesions: PCR swab of the lesion (not serology) is the test of choice. PCR is far more sensitive than viral culture.
• Recurrent genital symptoms suspicious for herpes but prior swab negative: Type-specific HSV IgG (HSV-1 and HSV-2 separately) can help confirm diagnosis.
• Partner of someone with known genital herpes: Type-specific serology can determine if the uninfected partner already has antibodies.
• HIV-positive patients: Serologic screening for HSV-2 is reasonable given increased risks.
• Pregnancy with unknown HSV status and partner with known genital herpes: Serology helps guide suppressive therapy decisions near delivery.

The Low-Index-Value Trap

An HSV-2 IgG index value of 1.1–3.5 has a high false-positive rate and should be confirmed with a supplemental assay (like the Biokit HSV-2 rapid test or the University of Washington Western blot, the gold standard). Do NOT diagnose a patient with genital herpes based on a low-positive HSV-2 IgG alone. The psychological and relational damage from a false diagnosis is real and lasting.

Trichomonas

NAAT is now the most sensitive test for Trichomonas vaginalis and is preferred over wet mount (which misses up to 50% of infections). Many GC/CT NAAT platforms now include trichomoniasis. Screen sexually active women with vaginal symptoms, and consider routine screening in high-prevalence populations (especially Black women, who have disproportionately high rates).

Screening Summary by Population

Population	What to Screen	How Often
All adults 13–64	HIV (4th-gen)	At least once
Sexually active women <25 (or ≥25 with risk factors)	GC/CT (urogenital NAAT)	Annually
MSM	HIV, syphilis, GC/CT (all 3 sites), HCV (if HIV+)	At least annually; every 3–6 months if high risk
Pregnant patients	HIV, syphilis (×3), HBV, GC/CT, HCV	First prenatal visit; repeat syphilis at 28 weeks and delivery
PrEP patients	HIV (4th-gen), syphilis, GC/CT (all sites), renal function	Every 3 months
Asymptomatic individuals	NOT: HSV serology, routine trichomoniasis	Do not screen routinely

Bottom Line

HIV testing has evolved—use the 4th-gen combo and stop relying on Western blot. GC/CT testing must be site-specific to be useful. Herpes serology should almost never be ordered routinely and causes more harm than good in asymptomatic patients. And the most commonly missed step in all of STI management: the 3-month retest after treating GC or CT.

Stay sharp out there.

Coagulation Studies: PT/INR, aPTT, D-dimer, and Fibrinogen Demystified

 

Coagulation Studies: PT/INR, aPTT, D-dimer, and Fibrinogen Demystified

When a prolonged aPTT is lupus anticoagulant vs. factor deficiency vs. heparin contamination—and why D-dimer is the most overordered test in the ED.

Coagulation studies get ordered constantly but are interpreted correctly far less often. The most common mistakes: ordering a D-dimer on a patient who clearly needs imaging, panicking about a mildly prolonged aPTT in a preop patient, and missing the lupus anticoagulant connection that ties directly into this series. Let's decode the panel.

The Tests and What They Measure

Test	Pathway	What It Assesses	Prolonged By
PT (Prothrombin Time) / INR	Extrinsic + common pathway (Factors VII, X, V, II, fibrinogen)	Warfarin monitoring; liver synthetic function	Warfarin, liver disease, vitamin K deficiency, DIC, factor VII deficiency
aPTT (Activated Partial Thromboplastin Time)	Intrinsic + common pathway (Factors XII, XI, IX, VIII, X, V, II, fibrinogen)	Heparin monitoring; intrinsic factor deficiencies; lupus anticoagulant screen	Heparin, factor deficiencies (hemophilia A/B), lupus anticoagulant, von Willebrand disease, DIC
D-dimer	Fibrinolysis	Detects fibrin degradation products	VTE, PE, DIC, surgery, trauma, pregnancy, malignancy, infection, inflammation, age
Fibrinogen	Common pathway	Clotting substrate; acute-phase reactant	Low in DIC, severe liver disease, massive hemorrhage. High in inflammation, pregnancy.

The Interpretation Patterns

PT/INR	aPTT	Think About
Prolonged	Normal	Warfarin therapy, early vitamin K deficiency, early liver disease, isolated factor VII deficiency
Normal	Prolonged	Heparin therapy, lupus anticoagulant, hemophilia A (factor VIII) or B (factor IX), von Willebrand disease, factor XI/XII deficiency, specimen contamination with heparin
Prolonged	Prolonged	DIC, severe liver disease, common pathway factor deficiency (X, V, II, fibrinogen), supratherapeutic anticoagulation, massive transfusion
Normal	Normal	Normal coagulation; does NOT exclude platelet disorders or von Willebrand disease (mild forms may have normal aPTT)

The Mixing Study: The Tiebreaker

When the aPTT is prolonged and the cause is unclear, a mixing study is the next step. The patient's plasma is mixed 1:1 with normal pooled plasma:

• aPTT corrects (normalizes with mixing): Factor deficiency—the normal plasma provides the missing factor. Investigate for hemophilia, vWD, or other factor deficiencies.
• aPTT does NOT correct: An inhibitor is present, blocking the coagulation pathway. The two most common inhibitors: lupus anticoagulant (the most common) and acquired factor VIII inhibitor (rare but serious).

The Lupus Anticoagulant Connection

This ties directly to the antiphospholipid antibody post in this series. Lupus anticoagulant (LA) paradoxically prolongs the aPTT in vitro but causes clotting in vivo. A prolonged aPTT that doesn't correct on mixing study + clinical thrombosis = suspect APS. Key points: LA interferes with phospholipid-dependent coagulation assays (hence the prolonged aPTT), but it's a pro-thrombotic condition. Never give a patient with suspected APS FFP or factor replacement for a "prolonged aPTT"—the prolongation is the antibody, not a bleeding diathesis.

D-dimer: The Most Overordered Test in Emergency Medicine

When D-dimer Is Useful

D-dimer has high sensitivity but low specificity for VTE. Its clinical value is as a rule-out test in low-to-moderate pretest probability patients:

• Low/moderate Wells score for PE or DVT + negative D-dimer = VTE excluded. No imaging needed.
• Age-adjusted D-dimer cutoffs (age × 10 for patients >50) improve specificity without sacrificing sensitivity.

When D-dimer Is Useless (or Harmful)

• High pretest probability: If the Wells score indicates PE/DVT is likely, skip the D-dimer and go straight to imaging (CT-PA or duplex ultrasound). A negative D-dimer in high-probability patients does NOT reliably exclude VTE.
• Hospitalized patients: D-dimer is elevated in almost every hospitalized patient (surgery, infection, inflammation, immobilization). It has almost zero specificity in this population.
• Pregnancy: D-dimer rises physiologically throughout pregnancy. Standard cutoffs don't apply. Imaging is generally preferred for suspected VTE in pregnancy.
• Active cancer, recent surgery, trauma, elderly patients: D-dimer is chronically elevated. A positive result tells you nothing useful.

The Harm of Overordering

Ordering D-dimer on a high-probability patient creates a dilemma: if it's negative (which it sometimes is even with PE), you might falsely reassure yourself. If it's positive (which it almost always is in sick patients), you've ordered a useless test that now mandates imaging anyway. Use clinical decision rules (Wells, PERC) first. D-dimer is for low/intermediate probability only.

D-dimer in DIC

In the context of sepsis or obstetric emergencies, a markedly elevated D-dimer with concurrent thrombocytopenia, prolonged PT/aPTT, and low fibrinogen = DIC. Here, D-dimer has a different role: it's part of the DIC score, not a VTE screen.

Fibrinogen: The Overlooked Marker

• Low fibrinogen (<200): DIC (consumed), severe liver disease (not synthesized), massive hemorrhage (diluted), rare congenital hypofibrinogenemia
• Very low fibrinogen (<100): Critical—risk of uncontrollable bleeding. Replace with cryoprecipitate.
• High fibrinogen: Acute-phase reactant. Elevated in infection, inflammation, pregnancy, malignancy. Also an independent cardiovascular risk factor.

DIC: The Pattern to Recognize

DIC is not a single lab result—it's a pattern across multiple tests in the right clinical context (sepsis, trauma, obstetric emergency, malignancy):

• Prolonged PT and aPTT
• Thrombocytopenia (platelets being consumed)
• Elevated D-dimer (fibrinolysis)
• Low fibrinogen (consumed)
• Schistocytes on peripheral blood smear (microangiopathic hemolysis)

Pediatric Note

Neonates have physiologically prolonged PT and aPTT due to immature clotting factor production (especially vitamin K-dependent factors). This normalizes by 6 months. Always use age-appropriate reference ranges for neonatal and pediatric coagulation studies. Vitamin K deficiency bleeding (formerly "hemorrhagic disease of the newborn") is prevented by routine vitamin K injection at birth—a critical reason to never skip it.

Preoperative Coagulation Testing: When It's Needed (and When It Isn't)

Routine preoperative PT/aPTT in patients with no personal or family history of bleeding and no clinical evidence of a bleeding disorder is not recommended per multiple Choosing Wisely guidelines. A thorough bleeding history is more informative than reflexive lab testing.

Order preoperative coagulation studies when: known bleeding disorder, anticoagulant use, liver disease, malnutrition, suspected malabsorption, or a positive bleeding history (easy bruising, heavy menses, prolonged bleeding after dental procedures or surgeries, family history of hemophilia or vWD).

The Pitfalls

• Heparin contamination: The most common cause of a falsely prolonged aPTT in hospitalized patients is a specimen drawn from a heparinized line. Always draw from a non-heparinized site or waste adequately before collecting.
• Assuming prolonged aPTT = bleeding risk: Lupus anticoagulant prolongs aPTT but causes thrombosis, not bleeding. Factor XII deficiency prolongs aPTT but has NO bleeding risk. The aPTT is not a bleeding predictor—it's a pathway assay.
• Using D-dimer as a screening test: D-dimer should only be ordered after calculating pretest probability. It is NOT a screening test for VTE.
• Forgetting that PT/INR reflects liver function: In a patient with no anticoagulant use, an elevated INR = impaired hepatic synthetic function until proven otherwise.
• Not checking fibrinogen in suspected DIC: Fibrinogen is often the forgotten component of the DIC workup. A falling fibrinogen is one of the most specific signs.
• Interpreting neonatal coag studies with adult norms: Neonates are physiologically "prolonged." Use neonatal reference ranges.

Bottom Line

PT measures the extrinsic pathway and liver synthetic function. aPTT measures the intrinsic pathway and is the test affected by heparin and lupus anticoagulant. D-dimer rules out VTE in low-probability patients and is useless in high-probability or hospitalized patients. Fibrinogen is the acute-phase reactant you're forgetting to check in DIC. And a prolonged aPTT that doesn't correct on mixing is lupus anticoagulant until proven otherwise—which means clotting, not bleeding.

Stay sharp out there.

LFTs Decoded: What's Actually "Liver" and What Isn't

 

LFTs Decoded: What's Actually "Liver" and What Isn't

The AST/ALT ratio, the hepatocellular vs. cholestatic pattern, and the muscle connection everyone misses.

"Liver function tests" is a misnomer that has confused generations of clinicians. Most of the tests in the panel don't measure liver function at all—they measure liver injury. True liver function is reflected by albumin, bilirubin, PT/INR, and the liver's synthetic capacity. The aminotransferases (AST and ALT) tell you about hepatocyte damage, not about how well the liver is doing its job.

Let's sort out what each test actually means.

The Tests: Injury Markers vs. Function Markers

Test	What It Measures	Key Interpretation
ALT (Alanine Aminotransferase)	Hepatocyte injury marker. More liver-specific than AST.	Elevated in hepatitis (viral, autoimmune, drug-induced, MASLD). The most specific "liver enzyme."
AST (Aspartate Aminotransferase)	Found in liver AND muscle, heart, kidney, brain, RBCs	Elevated in liver disease AND muscle injury, hemolysis, MI. Not liver-specific.
ALP (Alkaline Phosphatase)	Found in bile ducts, bone, placenta, intestine	Elevated in cholestasis (biliary obstruction) AND bone disease. Confirm hepatic origin with GGT.
GGT (Gamma-Glutamyl Transferase)	Biliary/hepatic enzyme	Confirms that elevated ALP is hepatic (not bone). Also elevated with alcohol use and enzyme-inducing medications.
Bilirubin (total and direct)	Product of heme breakdown	Direct (conjugated) elevated: hepatocellular disease or biliary obstruction. Indirect (unconjugated) elevated: hemolysis or Gilbert's syndrome.
Albumin	TRUE liver function marker (synthetic capacity)	Low in chronic liver disease, malnutrition, nephrotic syndrome, inflammation. Half-life 21 days—doesn't change acutely.
PT/INR	TRUE liver function marker (synthetic capacity)	Elevated when liver can't produce clotting factors. Responds within hours to days (unlike albumin). The best acute marker of liver synthetic failure.

The Two Patterns: Hepatocellular vs. Cholestatic

This is the most important framework for interpreting LFTs. When you see elevated liver enzymes, classify the pattern:

Pattern	AST/ALT	ALP/GGT	Think About
Hepatocellular	↑↑↑ Markedly elevated	Normal or mildly elevated	Viral hepatitis, drug-induced liver injury (DILI), autoimmune hepatitis, ischemic hepatitis ("shock liver"), Wilson's disease, acetaminophen toxicity
Cholestatic	Normal or mildly elevated	↑↑↑ Markedly elevated	Biliary obstruction (gallstones, tumor), primary biliary cholangitis, primary sclerosing cholangitis, drug-induced cholestasis
Mixed	↑ Elevated	↑ Elevated	Drug-induced, infiltrative diseases, granulomatous hepatitis, some viral hepatitis

The R Ratio

To formalize the pattern: R = (ALT/ALT ULN) ÷ (ALP/ALP ULN). R >5 = hepatocellular. R <2 = cholestatic. R 2–5 = mixed. This calculation helps when the pattern isn't immediately obvious.

The AST/ALT Ratio: Hidden Clues

• AST/ALT >2:1: Classic for alcoholic liver disease. The ratio occurs because alcohol selectively depletes hepatic pyridoxal-5-phosphate (needed for ALT production) and also causes mitochondrial injury (releasing more AST).
• AST/ALT ~1:1: Most other liver diseases (viral hepatitis, MASLD, drug-induced)
• ALT > AST: Typical of MASLD/MASH (fatty liver), chronic hepatitis C
• AST markedly elevated with relatively normal ALT: Think non-hepatic source—muscle injury (check CPK!), hemolysis, myocardial injury

The Myositis Connection (From Earlier in This Series)

Elevated AST with normal or mildly elevated ALT + elevated CPK = muscle, not liver. This was covered in the myositis antibody post and the Bellocchi autoimmune review. A patient referred for "elevated transaminases" who also has proximal weakness needs a CPK and a myositis workup—not a liver biopsy. Always check CPK when AST is disproportionately elevated.

The Magnitude Matters

ALT/AST Level	Differential
<5× ULN (mild)	MASLD/MASH, medications, alcohol, celiac disease, thyroid disease, hemochromatosis, alpha-1 antitrypsin deficiency
5–15× ULN (moderate)	Acute viral hepatitis, autoimmune hepatitis, drug-induced, alcoholic hepatitis, biliary obstruction
>15× ULN (severe / >1000)	Acetaminophen toxicity, ischemic hepatitis ("shock liver"), acute viral hepatitis (A, B, rarely C), autoimmune hepatitis flare, acute Budd-Chiari, Wilson's disease (acute presentation)

The Workup: What to Order Next

For Persistent Mild Elevation (Most Common Primary Care Scenario)

1. Confirm persistence: repeat in 3–6 months with fasting labs
2. Check: hepatitis B panel (HBsAg, anti-HBs, anti-HBc), HCV antibody, iron studies + ferritin (hemochromatosis), ANA + IgG level (autoimmune hepatitis), CPK (muscle source), TSH, celiac panel (tTG-IgA + total IgA), fasting lipid panel + HbA1c (metabolic syndrome/MASLD)
3. Imaging: RUQ ultrasound (assess for fatty liver, gallstones, structural lesions)
4. Review medications and supplements (statins, NSAIDs, acetaminophen, herbals, antibiotics)
5. Assess alcohol use honestly

For Acute Severe Elevation (>1000)

This is urgent. Check acetaminophen level, hepatitis A IgM, hepatitis B core IgM, HCV RNA, autoimmune hepatitis panel (ANA, anti-smooth muscle, IgG), toxicology screen, PT/INR (synthetic function), and Doppler ultrasound (Budd-Chiari, portal vein thrombosis). Consider Wilson's disease in young patients (ceruloplasmin, 24-hour urine copper).

Pediatric Considerations

In children with unexplained elevated transaminases, the differential includes everything above plus: Wilson's disease (screen any child >3 years with unexplained liver disease), alpha-1 antitrypsin deficiency, Reye syndrome (ASA + viral illness—now rare), metabolic liver diseases (galactosemia, tyrosinemia in infants), and celiac disease. Remember that neonatal "physiologic jaundice" is indirect bilirubinemia from immature conjugation—a direct bilirubin >1 mg/dL in a newborn is ALWAYS pathologic and warrants urgent evaluation (biliary atresia must be ruled out).

The Pitfalls

• Calling them "liver function tests" when AST/ALT/ALP are injury markers. True function = albumin + PT/INR + bilirubin.
• Assuming elevated AST = liver disease. AST is in muscle, heart, kidney, and RBCs. Check CPK.
• Not using the hepatocellular vs. cholestatic framework. The pattern determines the workup.
• Ignoring an isolated elevated ALP. Confirm hepatic origin with GGT. If GGT is normal, the ALP is from bone (Paget's, growing children, pregnancy, bone metastases).
• ALP is normally elevated in growing children (from bone growth plates)—a mildly elevated ALP in a healthy child is usually bone, not liver.
• Forgetting MASLD as the most common cause of mild transaminitis. It's now the most prevalent liver disease worldwide.
• Not checking celiac disease in unexplained transaminitis. Celiac causes cryptogenic elevation in up to 9% of cases.
• Stopping statins reflexively for mild ALT elevation. ALT <3× ULN on a statin can be monitored without discontinuation per current guidelines.

Bottom Line

LFTs are not one test—they're a panel of injury markers and function markers that tell very different stories. Classify the pattern (hepatocellular vs. cholestatic vs. mixed), consider the magnitude, always check CPK when AST is disproportionately high, and remember that albumin and PT/INR are the true measures of how well the liver is actually working. And never forget the two great masqueraders of liver disease: muscle injury and celiac disease.

Stay sharp out there.

The BMP Deep Dive: Eight Numbers That Tell You More Than You Think

 

The BMP Deep Dive: Eight Numbers That Tell You More Than You Think

Sodium algorithms, the anion gap you should be calculating on every panel, and why creatinine lies about half the time.

The basic metabolic panel is the most ordered lab in outpatient and inpatient medicine. Eight values: sodium, potassium, chloride, CO2 (bicarbonate), BUN, creatinine, glucose, and calcium. Most clinicians glance at the sodium and creatinine and move on. But buried in those eight numbers are patterns that diagnose DKA, renal failure, toxic ingestions, adrenal crises, and metabolic emergencies—if you know how to read them.

The Anion Gap: Calculate It Every Single Time

Anion Gap = Na − (Cl + CO2)

Normal: 4–12 mEq/L. This should be calculated on every BMP. An elevated anion gap means unmeasured acids are accumulating in the blood, and many of the causes are immediately life-threatening.

High Anion Gap Metabolic Acidosis (HAGMA)

The classic mnemonic is CAT MUDPILES:

• Cyanide, CO poisoning
• Aminoglycosides (rare)
• Toluene
• Methanol
• Uremia (renal failure)
• Diabetic ketoacidosis
• Propylene glycol, Paraldehyde
• Isoniazid, Iron
• Lactic acidosis (sepsis, shock, liver failure, metformin)
• Ethylene glycol
• Salicylates (aspirin overdose)

Red Flag

An anion gap >25 is almost always a true metabolic acidosis and demands immediate investigation. The most common causes in primary care and urgent care: DKA, lactic acidosis (sepsis), renal failure, and toxic ingestions. Don't dismiss it.

Normal Anion Gap (Non-Gap) Metabolic Acidosis

Low bicarbonate with a normal anion gap means chloride has risen to compensate (hyperchloremic acidosis). Think: diarrhea (most common), renal tubular acidosis, normal saline overresuscitation, carbonic anhydrase inhibitors (acetazolamide, topiramate), or ureteral diversions.

The Albumin Correction

Albumin is the major unmeasured anion. In hypoalbuminemic patients (cirrhosis, nephrotic syndrome, malnutrition, critical illness), the anion gap will be falsely low, masking a true HAGMA. For every 1 g/dL drop in albumin below 4, add 2.5 to the calculated anion gap. A patient with an albumin of 2 and a calculated AG of 10 actually has a corrected AG of 15—that's abnormal.

The Delta-Delta: Finding Hidden Disorders

When you find a HAGMA, go one step further:

• Δ Anion Gap = (Calculated AG) − 10
• Δ Bicarbonate = 24 − (Measured bicarb)
• If ΔAG ≈ ΔBicarb: pure HAGMA
• If ΔAG >> ΔBicarb: HAGMA + metabolic alkalosis (hidden vomiting or diuretic use)
• If ΔAG << ΔBicarb: HAGMA + non-gap metabolic acidosis (concurrent diarrhea or RTA)

Sodium: The Hydration and Osmolality Story

Hyponatremia (<135 mEq/L)

The most common electrolyte abnormality in hospitalized patients. The approach is stepwise:

1. Is it real? Rule out pseudohyponatremia (lab artifact from hyperlipidemia or hyperproteinemia on indirect ISE analyzers). Also correct for hyperglycemia: for every 100 mg/dL glucose above 100, add 1.6 to the sodium. If the corrected sodium is normal, the hyponatremia is dilutional from glucose, not true hyponatremia.
2. What's the volume status?
   • Hypovolemic (dehydrated): GI losses, diuretics, third-spacing. Sodium and water both lost, but sodium lost more. Treat with normal saline.
   • Euvolemic: SIADH (most common cause of euvolemic hyponatremia), hypothyroidism, adrenal insufficiency, beer potomania, tea-and-toast diet. Restrict fluids.
   • Hypervolemic (edematous): Heart failure, cirrhosis, nephrotic syndrome. Total body sodium is actually high but water is even higher. Restrict fluids + treat underlying cause.
3. How fast did it develop? Acute (<48 hours) and symptomatic hyponatremia (seizures, altered mental status) is an emergency requiring hypertonic saline. Chronic hyponatremia corrected too quickly risks osmotic demyelination syndrome—correct no faster than 8–10 mEq/L in 24 hours.

Pediatric Note

Hyponatremia in children is most commonly from GI losses (gastroenteritis) or from hypotonic fluid administration—using D5W or half-normal saline for maintenance fluids is a historical practice that has caused fatal hyponatremic encephalopathy. Current guidelines recommend isotonic fluids (normal saline or Lactated Ringer's) for maintenance IV fluids in children.

Potassium: The Cardiac Emergency Electrolyte

Hyperkalemia (>5.5 mEq/L)

Before you treat, rule out pseudohyperkalemia: hemolyzed specimen (most common), prolonged tourniquet time, or fist clenching during phlebotomy. If the patient is asymptomatic and the K+ is mildly elevated, repeat the draw.

True hyperkalemia causes: renal failure (most common), ACE inhibitors/ARBs/spironolactone, NSAIDs, rhabdomyolysis, tumor lysis syndrome, adrenal insufficiency, metabolic acidosis (H+ shifts into cells, K+ shifts out).

EKG Correlation Is Non-Negotiable

K+ >6.0 or any hyperkalemia with EKG changes (peaked T waves, widened QRS, sine wave pattern) is a cardiac emergency. Treat immediately with IV calcium (membrane stabilizer), insulin + glucose (shifts K+ intracellularly), and kayexalate or patiromer for definitive removal. EKG changes do NOT reliably correlate with absolute K+ level—some patients develop arrhythmias at 5.5, others tolerate 7.0. Always get the EKG.

Hypokalemia (<3.5 mEq/L)

Causes: diuretics (most common outpatient cause), GI losses (vomiting, diarrhea), renal losses, hypomagnesemia (must correct Mg before K will replete), alkalosis, insulin administration. EKG changes include flattened T waves, U waves, and prolonged QT—risk of torsades de pointes.

Clinical Pearl

Always check magnesium with potassium. Hypomagnesemia causes renal potassium wasting, and you cannot correct hypokalemia without first correcting the magnesium. This is one of the most commonly missed steps in electrolyte management.

BUN and Creatinine: The Renal Duo

Creatinine Pitfalls

Creatinine is the workhorse of renal function assessment, but it has significant blind spots:

• Muscle mass matters: A creatinine of 1.0 in a 90 lb elderly woman may represent significant renal impairment, while a creatinine of 1.4 in a muscular 25-year-old man may be normal. Always calculate eGFR (which adjusts for age, sex, and race).
• Creatinine lags behind injury: In acute kidney injury, creatinine doesn't rise until 24–48 hours after the insult. A "normal" creatinine in an acutely ill patient doesn't exclude AKI.
• Medications affect creatinine without affecting GFR: Trimethoprim and cimetidine block tubular secretion of creatinine, causing a rise that does NOT reflect true renal impairment.
• Cooked meat can transiently raise creatinine for 2–4 hours after a meal. Ideally, draw fasting.

BUN/Creatinine Ratio

Normal: 10–20:1. The ratio helps localize the cause of elevated creatinine:

• >20:1 (elevated BUN out of proportion): Prerenal azotemia (dehydration, heart failure, hemorrhage), GI bleeding (digested blood = protein load = BUN rises), high-protein diet, steroids
• <10:1 (creatinine disproportionately high): Intrinsic renal disease, rhabdomyolysis, muscle breakdown

Pediatric Note

Pediatric creatinine norms are much lower than adult values (neonates: 0.2–0.4 mg/dL, children: 0.3–0.7 depending on age/size). A creatinine of 0.9 in a 5-year-old is abnormal and may indicate significant renal impairment. Always use age-appropriate norms and pediatric eGFR equations (Schwartz formula).

Glucose

Fasting glucose ≥126 or random glucose ≥200 with symptoms = diabetes. But don't miss:

• Stress hyperglycemia: Acutely ill, septic, or post-surgical patients commonly have glucose 200–300+ without diabetes. This is a cortisol/catecholamine response. It should normalize with recovery but may unmask pre-diabetes.
• Hypoglycemia (<70) on BMP: In a non-diabetic patient, this is an emergency differential—consider insulinoma, adrenal insufficiency, liver failure, sepsis, alcohol use, or medication error.

Calcium

The BMP reports total calcium, which includes protein-bound calcium. In hypoalbuminemic patients, total calcium will be falsely low. Correct for albumin: corrected Ca = measured Ca + 0.8 × (4 − albumin). Or order an ionized (free) calcium, which is the physiologically active form and isn't affected by albumin.

• Hypercalcemia: The two most common causes account for ~90%: primary hyperparathyroidism (outpatient) and malignancy (inpatient). Check PTH: if elevated, it's hyperparathyroidism. If suppressed, it's humoral or osteolytic from cancer.
• Hypocalcemia: Vitamin D deficiency (most common), hypoparathyroidism (post-thyroidectomy), chronic kidney disease, hypomagnesemia (Mg is required for PTH secretion—another reason to always check Mg), pancreatitis.

CO2 (Bicarbonate)

This is the metabolic component of acid-base balance. The BMP reports total CO2, which is predominantly bicarbonate:

• Low CO2 (<22): Metabolic acidosis (calculate the anion gap!) or respiratory compensation for alkalosis
• High CO2 (>28): Metabolic alkalosis (vomiting, diuretics, volume contraction) or respiratory compensation for chronic respiratory acidosis (COPD)

The Pitfalls Summary

Pitfall	What Happens	How to Avoid
Not calculating anion gap	Missed DKA, lactic acidosis, toxic ingestion	Calculate AG = Na − (Cl + CO2) on every BMP
Not correcting AG for albumin	Missed HAGMA in hypoalbuminemic patients	Add 2.5 to AG for every 1 g/dL albumin below 4
Not correcting Na for glucose	Treating "hyponatremia" that's really hyperglycemia	Add 1.6 to Na for every 100 mg/dL glucose >100
Pseudohyperkalemia	Unnecessary treatment for hemolyzed specimen	Repeat draw if asymptomatic and no EKG changes
Not checking Mg with K	Refractory hypokalemia	Always check and correct Mg first
Trusting creatinine in small/elderly patients	Missed renal impairment	Always calculate eGFR; use age-appropriate peds norms
Not correcting calcium for albumin	False hypocalcemia in hypoalbuminemic patients	Corrected Ca = Ca + 0.8 × (4 − albumin), or order ionized Ca
Ignoring low CO2	Missed metabolic acidosis	Any bicarb <22 = calculate the anion gap

Bottom Line

The BMP is not eight independent numbers. It's a system. Sodium tells you about water balance and osmolality. Potassium tells you about cardiac risk and acid-base status. BUN and creatinine tell you about kidney function and volume status. Chloride and CO2 together reveal the anion gap. Glucose reveals metabolic control. Calcium reveals parathyroid function and albumin status. Read them together, calculate the anion gap every time, and correct for albumin and glucose when needed.

Eight numbers. One story. Read the whole thing.

Stay sharp out there.