Complement & Cryoglobulins: The Overlooked Labs That Connect Lupus, HCV, and Vasculitis

 

Complement & Cryoglobulins: The Overlooked Labs That Connect Lupus, HCV, and Vasculitis

Low C4 isn't always lupus. A false-negative cryoglobulin is almost always a specimen-handling error. Here's how to get these right.

Complement levels and cryoglobulin testing sit at the intersection of rheumatology, hepatology, hematology, and nephrology. They're ordered less frequently than ANA or RF, but when you need them, getting them right is critical—and getting the cryoglobulin specimen wrong means the test is useless before it even reaches the lab.

Part 1: Complement (C3 and C4)

What Are C3 and C4?

C3 and C4 are proteins in the complement cascade—part of the innate immune system that helps clear pathogens and immune complexes. When complement is "consumed" by active immune complex formation, serum levels drop. This consumption is a hallmark of active autoimmune disease, particularly SLE.

• C3 — Central to both the classical and alternative complement pathways. Low C3 is common in active SLE and is particularly associated with lupus nephritis.
• C4 — Part of the classical pathway (activated by immune complexes). Low C4 is seen in active SLE, cryoglobulinemia, and hereditary angioedema. C4 tends to drop before C3 in SLE flares.
• CH50 — Total hemolytic complement; measures the overall function of the entire classical pathway. Very low or undetectable CH50 suggests severe complement consumption or congenital complement deficiency.

The Pattern-Recognition Table

C3	C4	Think About
↓ Low	↓ Low	Active SLE (especially with nephritis); lupus flare; serum sickness
Normal	↓ Low	Cryoglobulinemia (classic pattern: low C4, relatively preserved C3); hereditary angioedema; early SLE flare (C4 drops first)
↓ Low	Normal	Alternative pathway activation: C3 nephritic factor, atypical HUS, membranoproliferative GN
Normal	Normal	Complement not consumed; does not exclude autoimmune disease (SACQ SLE, or disease in remission)
↑ High	↑ High	Acute-phase response (C3 and C4 are positive acute-phase reactants—they rise with inflammation, which can mask consumption)

Complement Pitfalls

Pitfall #1: Single Values vs. Trends

A single C3/C4 measurement is far less informative than serial trends over time. In SLE, longitudinal monitoring of complement is essential. A patient with a "normal" C3 that has dropped from 150 to 85 is very different from one with a stable C3 of 85. Always compare to baseline.

Pitfall #2: SACQ SLE

Serologically Active, Clinically Quiescent SLE is a state where complement is low and/or anti-dsDNA is elevated, but the patient has no clinical symptoms. About 30–50% of SACQ patients may eventually flare, but these isolated lab changes alone should not trigger treatment escalation. Complement must always be interpreted within the full clinical picture.

Pitfall #3: C3/C4 Are Acute-Phase Reactants Too

During active infection or inflammation, C3 and C4 production increases. This means a patient with active SLE AND a concurrent infection could have "normal" complement levels because inflammation-driven production is masking consumption. If the clinical picture doesn't match the labs, consider this possibility.

Part 2: Cryoglobulins

What Are Cryoglobulins?

Cryoglobulins are immunoglobulins that precipitate (form clumps) when cooled below body temperature. They deposit in small- to medium-sized blood vessels, causing vasculitis and end-organ damage. There are three types:

Type	Composition	Key Association	Classic Clue
Type I	Monoclonal Ig (usually IgM)	Lymphoproliferative disease (Waldenström's, myeloma, CLL)	Hyperviscosity symptoms; acral cyanosis; Raynaud's-like
Type II	Monoclonal IgM (RF activity) + polyclonal IgG	HCV infection (~80% of cases); also Sjögren's, SLE	Classic triad: palpable purpura, arthralgias, weakness
Type III	Polyclonal IgM (RF activity) + polyclonal IgG	Autoimmune diseases (SLE, RA, Sjögren's); infections	Milder vasculitis; may evolve to Type II over time

The HCV Connection

This cannot be overstated: HCV is the cause of mixed cryoglobulinemia in 70–90% of cases. Any patient with cryoglobulinemia must be tested for HCV. The clinical triad of palpable purpura, arthralgias, and weakness in an HCV-positive patient should immediately trigger cryoglobulin testing. Conversely, if your autoimmune patient has an unexplained positive RF, low C4, and purpura, always check HCV—mixed cryoglobulinemia may be the unifying diagnosis.

The Specimen Handling Pitfall (This Is Everything)

Critical: Warm Specimen Transport

Cryoglobulins precipitate at temperatures below 37°C. If the blood specimen cools during collection or transport, the cryoglobulins precipitate before the lab can detect them, causing a false-negative result. The proper procedure:

• Collect blood in a pre-warmed tube (37°C)
• Keep the specimen at 37°C during transport (use a warm water bath, heel warmer, or warm transport container)
• Allow the blood to clot and centrifuge at 37°C before cooling
• The serum is then incubated at 4°C for up to 7 days and checked daily for precipitate

If your lab cannot guarantee warm specimen handling, the cryoglobulin result is unreliable. Coordinate with the lab before ordering. A false-negative cryoglobulin is far worse than not testing at all, because it falsely reassures you.

The Diagnostic Clue Panel

When you suspect cryoglobulinemia, order all of the following together:

• Cryoglobulin screen (with warm transport!)
• C3 and C4 (expect very low C4 with relatively normal C3 in Type II)
• Rheumatoid factor (often elevated—mixed cryoglobulins have RF activity)
• HCV antibody + HCV RNA
• SPEP/UPEP with immunofixation (to assess for monoclonal component)
• Urinalysis (to screen for glomerulonephritis)
• Complement CH50 (very low in active cryoglobulinemic vasculitis)

More Cryoglobulin Pitfalls

• Low-level cryoglobulins (Type III) can take up to 7 days to precipitate. If your lab only incubates for 24–72 hours, milder cryoglobulinemias will be missed.
• A positive RF + low C4 + purpura in any patient should trigger cryoglobulin and HCV testing, regardless of whether you initially suspected cryoglobulinemia.
• "Essential" cryoglobulinemia is almost extinct as a diagnosis. Since the HCV association was discovered, most cases previously labeled "essential" are now attributed to HCV. If HCV testing is negative but clinical suspicion remains, check the cryoprecipitate itself for HCV RNA (it concentrates in the precipitate).
• Cryoglobulins can interfere with other lab tests. They can cause spurious results on CBC (pseudoleukocytosis, pseudothrombocytosis), protein electrophoresis, and complement levels if specimens are not handled at 37°C.

When Should NPs Order These Tests?

Order Complement (C3/C4) When You See

• Suspected SLE flare (with anti-dsDNA, CBC, urinalysis)
• Active lupus nephritis monitoring
• Suspected cryoglobulinemia (expect low C4)
• Recurrent angioedema without urticaria (hereditary angioedema: very low C4 between attacks)
• Membranoproliferative glomerulonephritis
• As part of the CTD workup when ANA is positive with clinical suspicion

Order Cryoglobulins When You See

• Palpable purpura (especially lower extremities) + arthralgias + weakness
• Unexplained low C4 with relatively normal C3
• Positive RF in an HCV-positive patient
• Glomerulonephritis with low complement (membranoproliferative pattern)
• Peripheral neuropathy + purpura + renal involvement
• Livedo reticularis or skin ulcers with vasculitic features
• Raynaud's-like symptoms with monoclonal gammopathy

Tying It All Together: The Autoimmune Series Connection

Complement and cryoglobulins connect to nearly every other topic in this series:

• SLE (from the autoimmune serologic testing review): Low C3/C4 are disease activity markers; serial trends matter more than single values
• Hepatitis panel: HCV drives 70–90% of mixed cryoglobulinemia; always check HCV when you find cryoglobulins
• Antiphospholipid syndrome: APS patients with SLE may have low complement from active lupus complicating the picture
• Rheumatoid factor: Elevated RF in a non-RA patient with low C4 and purpura should trigger cryoglobulin testing
• Syphilis screening: False-positive RPR in SLE patients may coexist with low complement during active flares

Bottom Line

Complement levels are simple to order but require clinical context to interpret. A single C3/C4 is a snapshot; serial trends tell the story. Low C4 out of proportion to C3 should make you think cryoglobulinemia before you think lupus. And cryoglobulin testing is only as good as the specimen handling—if the blood wasn't kept warm, the result doesn't count.

For your autoimmune patients, these labs are the bridge between rheumatology, hepatology, and hematology. Know when to order them, know how to interpret the patterns, and for cryoglobulins, know how to get the specimen right.

Stay sharp out there.

Iron Studies Made Simple: Pattern Recognition for the Busy NP

 

Iron Studies Made Simple: Pattern Recognition for the Busy NP

Four numbers, four patterns—and the one pitfall that makes ferritin lie to you every time.

Iron studies are ordered constantly in primary care, yet they're consistently misinterpreted. The most common mistake? Checking a ferritin alone and calling it a day. In your autoimmune, inflammatory, and chronically ill patients, a single ferritin can be profoundly misleading. Let's learn the patterns.

The Four Tests (and What Each Measures)

• Serum Ferritin — Reflects iron stores. The most sensitive early marker of iron deficiency when inflammation is absent. But it's an acute-phase reactant—levels rise with inflammation, infection, liver disease, obesity, and malignancy, masking true iron deficiency.
• Serum Iron — The amount of iron circulating in the blood, mostly bound to transferrin. Highly variable (diurnal fluctuation, affected by recent meals). Low in both iron deficiency AND anemia of chronic disease. Not useful in isolation.
• TIBC (Total Iron-Binding Capacity) — Reflects available transferrin binding sites. Essentially a proxy for transferrin levels. High in iron deficiency (body makes more transferrin to scavenge iron), low in inflammation/chronic disease (transferrin production suppressed by cytokines).
• Transferrin Saturation (TSAT) — Serum iron ÷ TIBC × 100. Tells you what percentage of transferrin's seats are occupied. Low (<20%) in iron deficiency. High (>45%) raises concern for iron overload.

The Key Principle

No single iron test is diagnostic on its own. Interpretation requires pattern recognition across all four tests, correlated with the clinical context (especially inflammation status). Always order the complete panel, not just ferritin.

The Patterns: Your Diagnostic Cheat Sheet

Condition	Ferritin	Serum Iron	TIBC	TSAT
Iron Deficiency Anemia	↓ Low (<30)	↓ Low	↑ High	↓ Low (<20%)
Anemia of Chronic Disease	↑ Normal/High	↓ Low	↓ Low/Normal	↓ Low/Normal
Mixed (IDA + ACD)	Normal (misleading!)	↓ Low	Variable	↓ Low
Iron Overload (Hemochromatosis)	↑↑ Very High	↑ High	↓ Low/Normal	↑ High (>45%)
Chronic Liver Disease	↑ High	↑ High	Variable	↑ High
Pregnancy (late)	↓ Low	↓ Low	↑ High	↓ Low

The Pitfalls

1. Ferritin Is a Liar (in Inflammatory States)

This is the single most important pitfall in iron studies. Ferritin is an acute-phase reactant. In your patients with RA, SLE, IBD, chronic infections, malignancy, obesity, or any active inflammatory state, ferritin can be normal or elevated despite true iron deficiency. A "normal" ferritin of 80 in an SLE patient with a CRP of 45 may be masking severe iron deficiency.

The Rule

In patients with active inflammation, use a ferritin cutoff of <100 µg/L (not <15 or <30) to detect iron deficiency, combined with a TSAT <20%. The WHO's traditional cutoff of 15 misses the majority of iron-deficient patients who have concurrent inflammation. Always check CRP alongside iron studies to know if inflammation is confounding the ferritin.

2. Serum Iron Fluctuates Wildly

Serum iron has up to 30% daily variation due to diurnal rhythm and dietary intake. It's highest in the morning and drops in the afternoon. A single random serum iron is nearly useless for diagnosis. Always draw iron studies fasting, in the morning, and rely on the full pattern rather than serum iron alone.

3. Anemia of Chronic Disease Looks Like Iron Deficiency (But Isn't)

Both conditions have low serum iron and low TSAT. The distinguishing feature: in ACD, ferritin is normal or high and TIBC is low or normal. In true iron deficiency, ferritin is low and TIBC is high. The body's iron isn't deficient in ACD—it's sequestered in macrophages by hepcidin, making it unavailable for erythropoiesis despite adequate stores.

4. Mixed Iron Deficiency + Chronic Disease: The Hardest Pattern

This is where it gets tricky. A patient with RA who also has iron deficiency from NSAID-related GI bleeding will have a ferritin that's been artificially inflated by inflammation, masking the deficiency. TSAT <20% with elevated CRP is your best clue. Soluble transferrin receptor (sTfR), when available, can help—it rises in true iron deficiency but is unaffected by inflammation.

5. Don't Treat Iron Overload Without Confirming

A high ferritin doesn't always mean iron overload. Exclude inflammation, liver disease, alcohol use, obesity, metabolic syndrome, and malignancy first. If TSAT is >45% AND ferritin is persistently elevated after excluding these causes, then pursue hereditary hemochromatosis workup (HFE gene testing).

6. Iron Deficiency Without Anemia Is Real

Fatigue, brain fog, restless legs, hair loss, and exercise intolerance can all occur with iron deficiency before the hemoglobin drops. A ferritin <30 with symptoms warrants treatment even if the CBC is normal. Don't wait for anemia to treat iron deficiency.

7. Always Ask "Why?"

Iron deficiency in a postmenopausal woman or any man requires GI evaluation until proven otherwise. Don't just hand them iron pills—look for the source of blood loss. Colon cancer, celiac disease, and H. pylori gastritis are all on the differential.

The Autoimmune Connection

For your autoimmune patients, iron studies are complicated by chronic inflammation at every turn:

• SLE, RA, IBD: Ferritin elevated by inflammation; use TSAT and higher ferritin cutoffs
• Celiac disease: Iron deficiency is often the presenting feature (malabsorption)
• Autoimmune gastritis: Pernicious anemia AND iron deficiency can coexist
• Chronic kidney disease: Functional iron deficiency (adequate stores but impaired utilization); guidelines use TSAT <20% and ferritin <100 as treatment thresholds
• Heart failure: Iron deficiency (TSAT <20% or ferritin <100) independently worsens outcomes; IV iron improves symptoms regardless of anemia status

Quick-Reference: When to Order and How to Interpret

Clinical Scenario	Order	Key Interpretation
Microcytic anemia	Full iron panel + CRP + CBC	Classic IDA: low ferritin, low iron, high TIBC, low TSAT
Anemia + active inflammation	Full iron panel + CRP	Ferritin may be falsely normal; rely on TSAT <20% and consider sTfR if available
Fatigue without anemia	Ferritin + TSAT	Ferritin <30 with symptoms = treat. Don't wait for anemia.
Elevated ferritin, unclear cause	Full iron panel + CRP + LFTs + HbA1c	Rule out inflammation, liver disease, metabolic syndrome before assuming overload
Suspected hemochromatosis	Fasting TSAT + ferritin	TSAT >45% + high ferritin after excluding secondary causes → HFE gene testing
Postmenopausal woman or any man with IDA	Full iron panel + GI referral	Investigate for source of blood loss. Colon cancer until proven otherwise.

Bottom Line

Iron studies are pattern recognition, not single-number diagnosis. Ferritin alone is not enough—especially in your inflammatory and autoimmune patients, where it lies to you constantly. Always order the full panel with CRP. Learn the four patterns (IDA, ACD, mixed, overload) and you'll interpret iron studies correctly in any clinical context.

Stay sharp out there.

Syphilis Screening: Traditional vs. Reverse Algorithms, False Positives, and the SLE Connection

 

Syphilis Screening: Traditional vs. Reverse Algorithms, False Positives, and the SLE Connection

Two tests, two algorithms, one disease that's surging back—and the autoimmune pitfall that keeps tripping everyone up.

Syphilis is back. Rates in the US have been climbing steadily, congenital syphilis cases have skyrocketed, and the CDC has made this a public health priority. For NPs in primary care, prenatal care, and urgent care, understanding the screening algorithms isn't optional anymore—it's essential. And the fact that your lab may be using a different algorithm than the one you learned in school makes it even more critical to know what you're looking at.

The Two Categories of Tests

Before we get to algorithms, you need to understand the two fundamentally different types of syphilis tests:

Nontreponemal Tests (RPR, VDRL)

• Detect antibodies to cardiolipin-cholesterol-lecithin antigens released from damaged host cells and from T. pallidum itself
• NOT specific to syphilis—they detect tissue damage, not the organism
• Reported as a quantitative titer (1:2, 1:4, 1:8, etc.)
• Titers correlate with disease activity and are used to monitor treatment response
• A fourfold change in titer (two dilutions, e.g., 1:16 to 1:4) = clinically significant change
• Become negative after successful treatment in most patients (though some remain "serofast" at a low titer indefinitely)
• Can be falsely positive in many conditions (see pitfalls below)

Treponemal Tests (FTA-ABS, TP-PA, EIA/CIA)

• Detect antibodies specific to Treponema pallidum
• Confirm that a nontreponemal positive is real syphilis (traditional algorithm)
• OR serve as the initial screen (reverse algorithm)
• Remain positive for life in 75–85% of patients after treatment—they are NOT useful for monitoring treatment response
• Cannot distinguish active from past/treated infection on their own
• False positives can occur with Lyme disease and other non-syphilis treponemal infections

The Two Algorithms

Traditional Algorithm

Screen with a nontreponemal test, confirm with a treponemal test.

1. RPR or VDRL (screening)
2. ↓ If reactive:
3. Treponemal test (FTA-ABS or TP-PA) to confirm
4. ↓ Both reactive = syphilis (stage based on clinical findings and titer)

Pros: Familiar, cost-effective, fewer false-positive workups in low-prevalence settings. RPR titer immediately available for staging and monitoring.
Cons: Can miss early primary syphilis and late latent syphilis (RPR may be nonreactive). Higher false-positive rate for the screening step.

Reverse Algorithm

Screen with a treponemal test, then get a nontreponemal titer.

1. Treponemal immunoassay (EIA or CIA—automated) as the screen
2. ↓ If reactive:
3. RPR (quantitative titer) to assess disease activity
4. ↓ If RPR is also reactive = syphilis (stage and treat)
5. ↓ If RPR is nonreactive (discordant):
6. Second, different treponemal test (TP-PA) as tiebreaker

Pros: Automated, high-throughput, catches latent syphilis and previously treated cases that RPR would miss. More sensitive overall.
Cons: Generates more discordant results requiring tiebreaker testing. Can identify past treated syphilis (treponemal tests stay positive for life), leading to unnecessary workups and overtreatment if not interpreted carefully.

Know Your Lab

Find out which algorithm your lab uses. Many high-volume commercial labs have switched to the reverse algorithm because of automation. If you're interpreting results without knowing which algorithm generated them, you can easily misread what a "positive syphilis screen" means. Ask your lab.

Interpreting the Results: The Cheat Sheet

Nontreponemal (RPR/VDRL)	Treponemal (FTA-ABS/TP-PA/EIA)	Interpretation
Nonreactive	Not done or nonreactive	No syphilis detected. (But can miss very early primary syphilis—both tests may be negative in the first 1–2 weeks of chancre.)
Reactive	Reactive	Syphilis confirmed. Stage and treat based on clinical findings and RPR titer. Could be active or past inadequately treated infection.
Reactive	Nonreactive	Biologic false positive RPR. The patient does NOT have syphilis. Investigate causes (see pitfalls).
Nonreactive	Reactive	Discordant (reverse algorithm scenario). Could be: (1) past treated syphilis (most common), (2) late latent syphilis with waning RPR, (3) very early primary syphilis, or (4) false-positive treponemal screen. Resolve with a second, different treponemal test (TP-PA).
Low-titer reactive (1:1–1:2)	Reactive	Could be serofast state (patient previously treated, RPR won't fully decline) or early/late infection. Correlate with treatment history and clinical exam.

The Pitfalls: Where NPs Get Tripped Up

1. The Biologic False-Positive RPR—and the SLE Connection

This is the pitfall that ties directly into the autoimmune series. The RPR and VDRL use cardiolipin as their antigen. Patients with antiphospholipid antibodies—including anticardiolipin antibodies—will have a false-positive RPR/VDRL because the test is detecting their autoantibodies, not a syphilis infection.

Conditions that cause biologic false-positive RPR/VDRL:

• SLE — the classic autoimmune culprit; up to 10–20% of SLE patients may have a false-positive RPR
• Antiphospholipid syndrome — by definition, these patients have anticardiolipin antibodies
• Pregnancy
• HIV infection
• IV drug use
• Other infections: hepatitis, EBV, malaria, tuberculosis, endocarditis
• Chronic liver disease
• Advanced age
• Recent vaccination

The Autoimmune Pearl

A positive RPR with a negative treponemal test in a young woman should make you think about SLE and antiphospholipid syndrome, not just dismiss it as a lab error. In fact, a biologic false-positive syphilis test was historically one of the earliest recognized clues to antiphospholipid antibody positivity. If you see this pattern, check an antiphospholipid panel (lupus anticoagulant, anticardiolipin, anti-β2GPI) and consider ANA testing.

2. The "Discordant Reverse Algorithm" Trap

With the reverse algorithm, you'll frequently encounter the scenario: treponemal screen reactive, RPR nonreactive. This generates a lot of confusion and unnecessary treatment. In most cases, this represents a patient with past, adequately treated syphilis whose treponemal antibodies persist for life. The key is the tiebreaker TP-PA:

• TP-PA reactive: The patient had real syphilis at some point. If no history of treatment, treat for late latent syphilis. If previously treated, no action needed (unless reexposure is suspected).
• TP-PA nonreactive: The original treponemal screen was a false positive. No syphilis. No treatment.

3. RPR Titers Are Not Interchangeable with VDRL

RPR and VDRL are both nontreponemal tests, but their titers are not directly comparable. An RPR of 1:8 and a VDRL of 1:8 may not represent the same antibody level. Always use the same test (and ideally the same lab) for serial monitoring. A fourfold change (two dilutions) is required to call a result clinically significant.

4. The Prozone Phenomenon

In secondary syphilis, antibody levels can be so high that they overwhelm the nontreponemal assay, causing a false-negative RPR (or a paradoxically low titer). This is called the prozone effect. If you have a patient with classic secondary syphilis symptoms (diffuse rash including palms/soles, mucous patches, condylomata lata) and a nonreactive or weakly reactive RPR, request that the lab perform serial dilutions to unmask a high-titer positive.

Clinical Pearl

If it looks like secondary syphilis clinically but the RPR is negative or suspiciously low, call your lab and ask for dilution testing. The prozone phenomenon is rare but real, and missing secondary syphilis has serious consequences.

5. Treponemal Tests Stay Positive for Life

Once treated, treponemal tests (FTA-ABS, TP-PA, EIA) remain reactive in 75–85% of patients forever. This does NOT indicate reinfection or treatment failure. Only 15–25% of patients treated during primary syphilis will eventually revert to seronegative. Do not use treponemal tests to monitor treatment response. Use the RPR titer.

6. Very Early Primary Syphilis Can Be Seronegative

In the first 1–2 weeks after a chancre appears, both nontreponemal and treponemal tests may still be negative. If you see a painless genital ulcer consistent with a chancre, treat empirically and retest in 2–4 weeks. Don't let a negative serology stop you from treating a clinical primary syphilis.

7. The Serofast State

Some patients, even after adequate treatment, will maintain a persistently low-titer RPR (typically 1:1 to 1:4) indefinitely. This is called the serofast state and does NOT indicate treatment failure or reinfection. It's more common in patients treated for late-stage syphilis. A serofast patient should be monitored but does not need retreatment unless there's clinical evidence of active disease or a fourfold rise in titer.

8. Pregnancy: Test Three Times

Given the surge in congenital syphilis, current guidelines (CDC, ACOG) now recommend screening pregnant patients three times: at the first prenatal visit, early in the third trimester (~28 weeks), and at delivery. This is non-negotiable. Congenital syphilis is devastating and entirely preventable with timely treatment.

When Should NPs Order Syphilis Testing?

Test When You See

• Any painless genital, anal, or oral ulcer (suspect primary syphilis)
• Diffuse rash involving palms and soles (suspect secondary syphilis)
• Mucous patches, condylomata lata, patchy alopecia
• All pregnant patients (three times during pregnancy)
• All patients diagnosed with another STI (HIV, gonorrhea, chlamydia)
• MSM (men who have sex with men)—per CDC, screen at least annually
• HIV-positive patients—screen at baseline and annually
• New sexual partner evaluation or sexual assault follow-up
• Unexplained neurologic symptoms in a patient with history of syphilis or risk factors
• Biologic false-positive RPR as part of a broader autoimmune workup (check aPL panel)

The SLE/APS Connection: A Summary for Your Autoimmune Patients

Tying It Together

In a patient with SLE or suspected antiphospholipid syndrome:

• A positive RPR/VDRL + negative treponemal test = biologic false positive, NOT syphilis. This is a clue to check antiphospholipid antibodies.
• Conversely, a positive treponemal test in an SLE patient should be taken seriously—the treponemal test is specific for syphilis and is NOT affected by antiphospholipid antibodies.
• The biologic false-positive syphilis test was historically one of the earliest recognized markers of antiphospholipid antibodies—before aPL assays existed, this was how clinicians first identified the autoantibodies that would later define antiphospholipid syndrome.
• Always confirm a reactive RPR with a treponemal test before diagnosing syphilis in an autoimmune patient. And always investigate a false-positive RPR for underlying autoimmune disease in a young patient.

Quick-Reference: The Decision Matrix

Scenario	What to Do
RPR reactive, treponemal reactive	Syphilis confirmed. Stage and treat. Monitor RPR titers.
RPR reactive, treponemal nonreactive	Biologic false positive. No syphilis. Investigate: SLE? APS? Pregnancy? Infection?
Treponemal screen reactive, RPR nonreactive (reverse algorithm)	Order TP-PA tiebreaker. If TP-PA reactive + no treatment history → treat for late latent. If TP-PA nonreactive → false-positive screen.
Previously treated, low-titer serofast RPR (1:1–1:4)	Monitor. No retreatment unless fourfold rise or clinical symptoms.
Clinical chancre, serology negative	Treat empirically. Retest in 2–4 weeks. Very early primary can be seronegative.
Pregnant patient	Screen at first visit, ~28 weeks, and delivery. Treat immediately if positive. Don't wait for confirmatory testing to treat in pregnancy.

Bottom Line

Syphilis testing is not complicated once you understand the two test types and the two algorithms. The critical points: know which algorithm your lab uses, never diagnose syphilis on a nontreponemal test alone, never use a treponemal test to monitor treatment, and always investigate a biologic false-positive RPR for underlying autoimmune disease. For your autoimmune patients, the RPR is a test that speaks two languages—infection and autoimmunity—and knowing which one it's speaking requires the treponemal test as the translator.

And with congenital syphilis surging, screen every pregnant patient three times. No exceptions.

Stay sharp out there.