By Dr Ananya Mandal, MD
Measles is most commonly diagnosed clinically based on signs and symptoms. There is a characteristic fever, rash and spots in the mouth that help in diagnosis of the condition.
Measles is a notifiable disease. If a doctor makes the diagnosis of measles, he or she is obliged to report it to the local Health Protection Unit and in case of a child, to the child’s school as well. This is done to prevent spread of the infection in the community.
Steps in the diagnosis of measles
Clinical symptoms including the characteristic rash and spots in the mouth are the primary step in diagnosing measles. In most cases this is enough to diagnose measles and isolate the affected person for at least 5 days after appearance of the rash to prevent spread of the infection to others.
Diagnosis that need to be ruled out – there are several conditions that may have similar features like measles. These need to be ruled out for confirmation of measles diagnosis. These include: –
For a confirmed diagnosis a swab is used to take a sample of the saliva or a blood sample is taken. The serum and saliva are measured for measles-specific immunoglobulin M (IgM). This is positive in cases of measles for up to 6 weeks after onset of the disease. Urine samples may also yield the virus and the IgM. Serologic testing is most commonly by enzyme-linked immunoassay (ELISA or EIA). ELISA for IgM antibody requires only a single serum specimen and is diagnostic if positive.
After an infection the IgM shows an immediate rise that persists for a month or two. This is diagnostic of measles. There are, however, tests to detect measles specific IgG as well. The tests include ELISA, hemagglutination inhibition (HI), indirect fluorescent antibody tests, microneutralization, and plaque reduction neutralization. These are diagnostic only if a rise in the IgG can be showed. This is called the rise in titer of antibody against measles virus. Thus at least two samples of serum need to be collected. The first specimen should be drawn as soon after rash onset as possible. The second specimen should be drawn 10–30 days later. Both samples should have the same test at the same time. The specific criteria for documenting an increase in titer depend on the test.
Measles virus contains a single stranded RNA in its core. These RNA strands may be detected in saliva and other samples for confirmation of diagnosis.
Isolation of measles virus is not recommended as a routine method to diagnose measles. However, studies of these isolates help in molecular epidemiologic surveillance of measles and help in detecting the geographic origin of the virus and strains of the virus. The virus is more likely to be isolated when the specimens are collected within 3 days of rash onset.
Further tests include those with buccal scrapings. This includes gently scraping the cells of the inner linings of the cheeks. These cells are then placed over a slide and stained with special dyes called Leishman's stain. Under the microscope giant cells are visible.
Yet another method is to use special fluorescent dyes and with a process called immunofluorescence of a nasopharyngeal aspirate (NPA) the virus may be detected.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)