What laboratory tests confirm the diagnosis of measles and erythema infectiosum in a patient?

Laboratory Confirmation of Measles and Erythema Infectiosum

For measles, obtain serum measles-specific IgM antibody testing using the direct-capture EIA method during the first clinical encounter, and for erythema infectiosum (parvovirus B19), obtain serum parvovirus B19-specific IgM antibody testing. 1

Measles Laboratory Diagnosis

Primary Diagnostic Test: Serum IgM

• Collect blood for measles IgM during the first clinical encounter with suspected measles, as IgM may be detectable at rash onset, peaks at approximately 10 days after rash onset, and remains detectable for 30-60 days. 1, 2
• The direct-capture IgM EIA method is the most sensitive and specific test for measles confirmation. 1
• If measles IgM is not detected in serum obtained within the first 72 hours after rash onset, collect a second specimen at least 72 hours after rash onset. 1, 2

Alternative Diagnostic Methods

• Paired acute and convalescent-phase serum specimens can demonstrate a significant rise in measles antibody titer: collect the acute-phase specimen within 1-3 days after rash onset and the convalescent-phase specimen approximately 2-4 weeks later. 1
• For IgG assays, a fourfold rise in antibody titer indicates recent infection. 3
• Virus isolation from clinical specimens (urine or nasopharyngeal mucus) should be collected as close to rash onset as possible. 1

Critical Pitfalls in Measles IgM Interpretation

• False-positive IgM results can occur with parvovirus B19 infection, acute infectious mononucleosis, cytomegalovirus, or in persons who are rheumatoid factor positive. 3, 2
• In low-prevalence settings, the likelihood of false-positive IgM results increases significantly. 2
• Recent measles vaccination (especially within 8 weeks) complicates interpretation: 73% of vaccinated infants remain IgM-positive at 4 weeks, declining to 52% at 5 weeks and only 7% by 8 weeks post-vaccination. 4
• Confirmatory testing using the direct-capture IgM EIA method is recommended when IgM is detected in a patient with no identified source of infection and no epidemiologic linkage to a confirmed case. 2

Erythema Infectiosum (Parvovirus B19) Laboratory Diagnosis

Primary Diagnostic Test: Serum IgM

• Serum parvovirus B19-specific IgM antibody testing confirms erythema infectiosum, with 84% (16 of 19) of sera from patients with erythema infectiosum testing positive by IgM. 5
• Parvovirus B19 IgM is the standard serologic method for acute infection confirmation. 6

Molecular Testing

• Multiplex RT-PCR can detect parvovirus B19 with sensitivity as low as one molecule in dilution experiments, making it highly sensitive for diagnosis. 5
• Real-time PCR detected parvovirus B19 in 14% (136 of 970) of suspected measles/rubella cases that tested negative for those viruses. 7

Clinical Context for Testing

• Erythema infectiosum is frequently misdiagnosed as measles or rubella due to similar clinical presentations with fever and rash. 7, 6
• Among suspected measles/rubella cases in Pernambuco, Brazil, parvovirus B19 accounted for 3.3% of positive diagnoses, making it the third most common etiology after dengue (16.9%) and before rubella (2.8%). 6
• In Osaka, Japan, 14% of measles/rubella-negative cases were confirmed as parvovirus B19 infection, with 64% occurring in adults (20 years and older). 7

Practical Testing Algorithm

Initial Approach

• Collect serum for both measles IgM and parvovirus B19 IgM simultaneously when evaluating fever-rash illness, as clinical differentiation is unreliable. 7, 6
• Laboratory confirmation should be attempted for every suspected case, with immediate notification of local health departments. 1
• Control activities should not be delayed pending laboratory results. 1

Timing Considerations

• For measles: optimal specimen collection is at rash onset through 30-60 days after onset. 1, 2
• If initial measles IgM is negative within 72 hours of rash onset, repeat testing after 72 hours. 1, 2
• For parvovirus B19: IgM typically becomes detectable when the characteristic "slapped cheek" rash appears, as viremia peaks before rash onset. 5

When Molecular Testing Is Indicated

• Multiplex RT-PCR can simultaneously detect measles virus, rubella virus, and parvovirus B19, providing definitive diagnosis when serologic results are equivocal or when rapid differentiation is needed. 5
• PCR is particularly useful for pharyngeal exudates, urine samples, and cerebrospinal fluid in complicated cases. 5