Measles (Rubeola)

Any patient who presents with a clinical syndrome consistent with measles, irrespective of potential exposure or travel history, should be evaluated for infection.1 While most measles cases have occurred in nonimmunized or incompletely immunized individuals, there also have been documented cases in those previously vaccinated.¹

Clinically, the diagnosis of measles is supported if Koplik’s spots are detected and if the rash progresses from the head to the trunk and out to the extremities. On average, the rash appears about 14 days after exposure to the virus, typically within a few days after onset of symptoms including fever, runny nose, congestion, cough, conjunctivitis, and/or malaise. The incubation period is 7 to 21 days. Affected patients are contagious 4 days prior to rash development and up to 4 days after the rash appears.^1,2

Public health authorities are asking healthcare providers to notify them directly if they have a patient with suspected measles.¹ This allows for tracking of highly suspect cases and the provision of appropriate public health follow-up. For highly suspect cases, local public health departments will recommend appropriate testing procedures.¹

Detection of measles-specific IgM antibody in serum and measles RNA by real-time polymerase chain reaction (RT-PCR) in a respiratory specimen are the most common methods for confirming measles infection. Measles RNA by RT-PCR is the preferred method for confirming an acute case.¹

A positive RT-PCR test confirms the measles diagnosis.^1,3 While detection of IgM antibody can be diagnostic, false-positive results may occur, especially in low-prevalence populations.¹ Conversion of a negative IgM result to a positive result or a 4-fold or greater increase in measles IgM titer between acute and convalescent serum specimens is diagnostic.¹ False-negative IgM results can also occur if the serum sample is obtained <3 days after rash presentation, (ie, prior to IgM antibody development). Additionally, IgG seroconversion can also help diagnose recent measles infection in the absence of recent measles vaccination. Laboratory confirmation is essential for all sporadic measles cases and all outbreaks.¹

Collect throat or nasopharyngeal swab samples as soon as a measles disease is suspected. The RT-PCR test has the greatest diagnostic sensitivity when samples are collected as soon as possible in a suspected measles patient.

Additionally, collect the first (acute phase) measles IgM/IgG serum sample as soon as possible upon disease presentation. If the acute-phase measles antibody sample collected ≤3 days after rash onset is negative, and the patient has a negative (or not yet reported) result for measles RT-PCR, a second serum sample 3 to 10 days after symptom onset may be useful. In some cases, the IgM measles antibody may not be detectable until 24 hours after symptoms appear.²

Detailed specimen collection instructions for measles testing can be found in the Test Directory at TestDirectory.QuestDiagnostics.com.

The reference range for the Measles Antibody (IgG) assay (test code 964[X]) has been lowered to improve sensitivity in accordance with recommendations from the World Health Organization (WHO) (Table).⁴ These revised assay cutoffs provide an estimated increase in positivity rate of 5% to 8%. This assay has a sensitivity of 94.7% and a specificity of 97.4%.⁵