ImmunologyTier 1Medical Sciences concept

Causes of false-positive serological testing (cross-reactivity, IVIG, pregnancy)

Core concept

PPV falls as prevalence falls - in a low-prevalence population, most positives from a good test are still false
Four mechanisms
- Cross-reactivity - a shared or similar epitope (cardiolipin, glycolipid, carbohydrate)
- Polyclonal B-cell activation - infection, autoimmunity, pregnancy generate low-affinity antibody that binds the assay antigen
- Passively acquired antibody - IVIG, blood products, recent vaccination, transplacental maternal IgG
- Assay interference - heterophile/human anti-mouse antibodies, rheumatoid factor, biotin, hook effect, haemolysis/lipaemia

Syphilis - the paradigm

	Non-treponemal (RPR, VDRL)	Treponemal (TPPA/TPHA, EIA, FTA-ABS)
Antigen	Cardiolipin - not a treponemal antigen at all	T. pallidum-specific
Use	Titre, tracks activity and response	Confirms exposure
After treatment	Falls (4-fold drop = response)	Stays positive for life

Biological false-positive RPR/VDRL - typically low titre (<1:8)
- Pregnancy, autoimmune disease (SLE, antiphospholipid syndrome), acute viral infection (EBV, hepatitis, HIV, measles), IV drug use, malaria, TB, leprosy, older age, recent vaccination, chronic liver disease
- A persistent biological false-positive VDRL is a recognised presenting feature of APS - a positive test can lead you to the right diagnosis by the wrong route
The prozone (hook) effect - very high antibody in secondary syphilis or HIV co-infection gives a falsely NEGATIVE undiluted RPR; the lab must dilute
Reverse-sequence algorithm (treponemal EIA first) throws up EIA+ / RPR- / TPPA- discordance -> usually a false-positive EIA, occasionally very early or long-treated infection

Other classic false positives

Test	Causes
ANA	Up to 15-30% of healthy people at 1:40; ~5% at 1:160, rises with age; infection, malignancy, drugs (hydralazine, procainamide, minocycline, anti-TNF)
RF	Sjogren's, endocarditis, hepatitis C, ~10-25% of healthy over-70s
ANCA	Infective endocarditis, TB, cocaine/levamisole, drug-induced (propylthiouracil, hydralazine), IBD - atypical/perinuclear patterns
Anticardiolipin / lupus anticoagulant	Acute infection - must be repeated at >=12 weeks; anticoagulants interfere with LA testing
D-dimer	Pregnancy, age, malignancy, surgery, infection, inflammation
Troponin	CKD, sepsis, PE, myocarditis, tachyarrhythmia, macrotroponin and heterophile antibody
hCG	Heterophile antibody -> phantom hCG (urine test is negative, which is the check)
Toxoplasma/CMV/rubella IgM	Persist for months; polyclonal activation - use IgG avidity to date infection
HIV screening EIA	Autoimmune disease, pregnancy, vaccination -> confirm with antigen/antibody differentiation assay

IVIG transfers donor antibody -> transient positive hepatitis, measles, CMV, EBV, toxoplasma and syphilis serology for up to 6 months; also causes a positive DAT and pseudo-hyponatraemia
Biotin (high-dose supplements) interferes with streptavidin-biotin immunoassays -> spuriously low TSH with high free T4, and falsely low troponin; ask about supplements and repeat after 48 h off

Confirm with a test of a different format or antigen, not a repeat of the same one
Order serology only when pre-test probability justifies it - "screening ANA" in fatigue creates more disease than it finds
Take blood BEFORE giving IVIG or blood products when serology matters
Repeat antiphospholipid antibodies at 12 weeks - single positives after infection are common and do not classify
In a discordant result, ask what the assay actually measures before deciding the patient has the disease

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