When should I order serology (IgM/IgG) tests and how should I interpret the results?

When to Order and How to Interpret Serology (IgM/IgG) Tests

Serology testing should be ordered primarily when acute infection is suspected beyond the early viremic phase (typically >5-7 days after symptom onset), when nucleic acid amplification tests (NAATs) are negative but clinical suspicion remains high, or for determining past exposure and immunity status. 1

Timing of Serological Testing

Acute Infection Diagnosis

• Order IgM/IgG serology 7-10 days after symptom onset for optimal sensitivity, as antibodies require time to develop after initial infection 1
• For dengue virus specifically, IgM antibodies become detectable 3-5 days after symptom onset and persist for 2-3 months, while IgG appears 10-12 days post-onset 1
• If initial serology is negative but clinical suspicion remains high, repeat testing on a convalescent sample collected 7-10 days later to detect seroconversion 1
• For SARS-CoV-2, serological testing should ideally be performed 21 days after potential infectious contact, as the majority of exposed individuals will have seroconverted by this time 2

When NAATs Are Preferred Over Serology

• NAATs are the primary diagnostic method during the acute viremic phase (first 4-6 days of illness for most viral infections) 1
• Serology serves as a complementary tool when NAATs are negative but false-negative results are suspected 2
• For respiratory pathogens like pertussis, NAATs combined with culture are now the recommended tests of choice, replacing older methods 1

Interpreting IgM and IgG Results

IgM Interpretation

IgM positivity suggests recent or acute infection, but false positives are common and must be interpreted with caution. 3

• False-positive IgM results occur frequently, particularly in populations with low disease prevalence 1, 3
• For hepatitis A, all confirmed acute infections had anti-HAV IgM values >4.0, and low-level reactive results (<4.0) were associated with alternative diagnoses in 63.6% of cases 3
• IgM can persist for months after acute infection, complicating interpretation of single time-point testing 1, 4
• In secondary dengue infections, IgM antibodies may not be detectable at all 1
• For EBV, up to 10% of Monospot tests yield false negatives, especially in younger children 1

IgG Interpretation

• IgG positivity indicates past infection or immunity (either from natural infection or vaccination) 1
• For acute infection diagnosis, demonstrate a four-fold or greater rise in IgG titer between acute and convalescent samples 1
• Single IgG measurements are generally not diagnostic of acute infection unless paired with clinical context and age-appropriate interpretation 1
• IgG antibodies persist for months to years after infection 1

Two-Tier Testing Strategy

For Lyme disease, always use two-tier testing: initial EIA screening followed by confirmatory Western blot only if EIA is positive or equivocal. 1

• Never perform Western blot without a positive or equivocal EIA result 1
• This approach reduces false-positive results and improves specificity 1

Critical Pitfalls and Caveats

Cross-Reactivity Issues

• Flavivirus antibodies cross-react extensively - false-positive dengue serology can occur in patients with prior West Nile virus, St. Louis encephalitis virus, or Zika virus infection 1
• Plaque reduction neutralization tests (PRNTs) provide improved specificity but are only available at select public health laboratories 1
• For Ehrlichia and Anaplasma, cross-reactivity occurs with Rocky Mountain Spotted Fever, typhus, Q fever, and Lyme disease antibodies 1

Timing-Dependent Sensitivity

• Serology is insensitive in the first 2 weeks of Lyme disease when erythema migrans is present 1
• For SARS-CoV-2, sensitivity reaches 100% only at ≥15 days post-PCR positivity when testing both IgM and IgG 5
• Median time to seropositivity for SARS-CoV-2 is 4-5 days from positive PCR, but this varies by individual 5

Special Populations

• Immunocompromised patients may not mount adequate antibody responses, making serology unreliable 1
• In pregnancy, negative maternal CMV IgM in the second trimester cannot exclude congenital CMV infection - consider CMV PCR on amniotic fluid if fetal abnormalities are detected 6
• For congenital infections, CMV IgG avidity testing on first-trimester samples helps distinguish primary from non-primary maternal infection 6

Specific Clinical Scenarios

Suspected False-Negative NAAT

• Order serology when clinical presentation strongly suggests infection but NAAT is negative 2
• Combine IgM and IgG testing to maximize sensitivity 2, 5
• Consider that NAAT sensitivity depends on specimen quality and timing of collection 1

Epidemiological Surveillance

• Serology is valuable for population screening and seroprevalence studies 2
• Can guide vaccination planning by identifying susceptible populations 2
• For TORCH agents in women of fertile age, screening identifies those at risk for congenital infections 7

Confirming Immunity

• Single IgG measurement is appropriate for determining immunity status (e.g., rubella, hepatitis A) 1
• For JC virus, FDA-cleared serologic screening (STRATIFY JCV) identifies patients at elevated risk of progressive multifocal leukoencephalopathy before starting natalizumab 1

Neuroinfections

• For neuroborreliosis, calculate CSF/serum antibody index using paired samples to demonstrate intrathecal antibody production 1
• This approach is more specific than serum serology alone for CNS involvement 1