How can a definitive diagnosis of measles be made in an adult presenting with a rash suggestive of measles?

From the Guidelines

To definitively diagnose measles in an adult with a suggestive rash, laboratory confirmation through serologic testing for measles-specific IgM antibodies is essential, as it can be detected as soon as possible after rash onset using a sensitive and specific assay, such as the direct-capture IgM EIA method 1. The timing of specimen collection is crucial, as IgM antibody may not be detectable until at least 72 hours after rash onset, and if not detected in the first 72 hours, another specimen should be obtained at least 72 hours after rash onset and tested for measles IgM antibody 1. Key points to consider in the diagnostic process include:

• Collecting blood for serologic testing during the first clinical encounter with a person who has suspected or probable measles 1
• Interpreting serologic data based on the timing of specimen collection in relation to rash onset and the characteristics of the antibody assay used 1
• Considering alternative diagnoses, such as rubella, in persons with febrile rash illnesses who are seronegative for measles 1
• The use of a significant rise in antibody titer between acute- and convalescent-phase serum specimens as a method for serologic diagnosis, although this has been largely supplanted by IgM assays 1. While clinical features such as prodromal symptoms, presence of Koplik spots, and the characteristic maculopapular rash can support the diagnosis, laboratory confirmation is crucial due to the potential for other conditions to mimic measles. Prompt reporting of suspected cases to public health authorities is also necessary, given the highly contagious nature of measles.

From the Research

Diagnostic Methods for Measles

To make a definitive diagnosis of measles in an adult with a suggestive rash, several laboratory methods can be employed. These include:

• IgM detection: This method involves detecting IgM antibodies against the measles virus in serum samples. According to 2, IgM detection shows the highest sensitivity when serum samples are collected ≥5 days after rash onset.
• Virus isolation: This method involves isolating the measles virus from clinical specimens such as nasopharyngeal aspirate, urine, or throat/combined throat and nasal swab. As reported in 2, virus isolation gives the highest sensitivity when samples are collected ≤3 days after rash onset.
• RT-PCR: This method involves detecting measles virus RNA in clinical specimens using reverse transcriptase polymerase chain reaction. According to 2 and 3, RT-PCR is a highly sensitive method for diagnosing measles, especially when used with peripheral blood mononuclear cell (PBMC) samples or throat swabs.

Specimen Collection and Timing

The timing and type of specimen collection are crucial for accurate diagnosis. The optimal timing for specimen collection varies depending on the diagnostic method used:

• For IgM detection, serum samples should be collected ≥5 days after rash onset 2.
• For virus isolation, samples should be collected ≤3 days after rash onset, with nasopharyngeal aspirate being the best specimen type, followed by urine and throat/combined throat and nasal swab 2.
• For RT-PCR, the highest positive rate is obtained with serum samples collected ≤3 days after rash onset, or with throat/combined throat and nasal swab, urine, and nasopharyngeal aspirate collected ≤16,4-16, and 4-7 days after rash onset, respectively 2.

Comparison of Diagnostic Methods

The sensitivity and specificity of different diagnostic methods for measles have been evaluated in several studies:

• 3 reported that RT-PCR using PBMC samples is the most effective method for diagnosing measles, with a sensitivity of 100% and a specificity of 91.7%.
• 4 compared the diagnostic performance of four commercially available IgM tests and found that they all displayed significant agreement and high diagnostic accuracy in primary infection, but lower accuracy in suspected reinfection.
• 5 developed and validated real-time RT-PCR assays for detecting measles virus on the Hologic Panther Fusion® System, which showed 100% clinical sensitivity and specificity.
• 6 evaluated the diagnostic accuracy of eight commercial assays for detecting measles virus-specific IgM antibodies and found that the sensitivities and specificities ranged from 75.0% to 98.1% and 86.6% to 99.5%, respectively.