Clinical Use

Diagnose measles (rubeola) infection

Clinical Background

Measles (rubeola) is a highly contagious, acute viral illness. It is characterized by a febrile rash that appears about 14 days after exposure to the virus and follows fever, malaise, Koplick spots, and the 3 "C"s: cough, coryza (runny nose), and conjunctivitis. The Centers for Disease Control and Prevention (CDC) reports a recent resurgence in measles infection, owing to transmission from individuals from areas where disease is endemic, and decreased vaccination rates related to the anti-vaccination movement.1,2 Antiviral therapy is not available for measles, but timely diagnosis may allow clinicians to initiate supportive care sooner and limit transmission.3

The CDC recommends 2 complementary testing methods for diagnosing suspected measles infection: IgM antibody testing and real-time polymerase chain reaction (real-time PCR).3-5 IgM antibodies may be detected 3 days after rash onset; however, when testing is done too early, IgM levels can be low and cause false-negative results.6 False-positive IgM results can also occur owing to parvovirus B19, Epstein-Barr virus, and other viruses.7

Real-time PCR addresses some constraints of IgM antibody testing.4,5,8 Real-time PCR testing can detect viral RNA before IgM becomes detectable and can thus support a measles diagnosis earlier in disease progression.4,9 Furthermore, real-time PCR can detect nucleotide sequences specific to measles RNA, thus avoiding false-positive results caused by the detection of IgM antibodies to other viruses.7

Real-time PCR also performs better than viral culture. The sensitivity of culture is greatest for specimens collected within 3 days after onset of febrile rash (65% sensitivity) but falls below 10% after 10 to 14 days.5,6,8 In contrast, the sensitivity of real-time PCR for measles is 80% within 3 days of rash onset and remains above 50% as late as 10 to 14 days after onset.5,6,8

Individuals Suitable for Testing

Individuals who present with symptoms of measles infection (ie, febrile rash, cough, coryza, conjunctivitis), especially those who are unvaccinated or have other risk factors such as recent international travel
Method

Real-time PCR-based amplification of extracted nucleic acids
Interpretive Information

A "detected" result confirms a measles diagnosis.4

A "not detected" result is consistent with the absence of measles infection but does not exclude the diagnosis, because real-time PCR is affected by the timing of specimen collection and other factors. If measles is suspected and rubeola virus RNA is not detected, consider measles IgM/IgG testing.

Diagnosis of measles infection should not rely solely on the result of this test. Test selection and interpretation, diagnosis, and patient management decisions should be based on the physician's education, clinical expertise, and assessment of the patient.

Methodology
Immunoassay (IA)

Reference Range(s)
AU/mL    Interpretation
<13.50    Not consistent with immunity
13.50-16.49    Equivocal 
>16.49    Consistent with immunity The presence of measles IgG suggests immunization or past or current infection with measles virus.

Alternative Name(s)
Rubeola, IgG Antibodies,Measles, Immune Status