TABLE 2. Summary of laboratory diagnostics for measles.

LABORATORY DIAGNOSTIC ELEMENT	DESCRIPTION
Specimen collection	Upon suspicion of measles, clinicians should immediately collect specimens for serology and virus detection for the purpose of laboratory confirmation. For viral detection, nasopharyngeal (preferably) or throat swabs (or washes) should be collected as soon as possible and no later than 4 days from the onset of rash. Measles virus may be still detected after 7 days from the onset of rash, but with rapidly decreasing sensitivity. Specimens should be collected using a swab approved for virus isolation and placed in virus transport media. Although nasopharyngeal specimens are preferred, the measles virus can also be detected in urine, which should be collected within 7 days from the rash onset, for maximum sensitivity. For serological testing, a serum specimen should be collected as soon as possible, when the patient is first seen. For IgM serology, a sample collected before 3 days and after 28 days from rash onset may yield a false negative result. For IgG serology, the first (acute) sample should be collected no later than 7 days from rash onset and a second (convalescent) sample 10 to 30 days after the first.
Serology	Presence of measles-specific IgM-class antibody is indicative of acute measles infection when rash was present and there is a history of exposure to measles through travel to an endemic area or an epidemiological link to a confirmed case. Positive IgM results not associated with acute measles infection may be due to the following reasons: • A positive measles IgM result in a sporadic case of rash without a history of exposure is possibly a false positive and it should be carefully investigated. Anti-measles IgM are frequently elevated in patients who received the MMR vaccine within 6 weeks prior to rash onset. Negative IgM results in a true measles case may also occur: • if the specimen is taken from the patient earlier than 3 days or later than 28 days after rash onset. • The immune response to measles infection in previously immunized individuals or those with pre-existing immunity may not be typical of the response in a measles naïve individual, and such cases may not have an IgM response. Virological confirmation and/ or acute and convalescent IgG testing should be conducted to confirm the infection in such cases. Seroconversion (i.e. negative to positive result) or a four-fold or greater rise in IgG titre between the acute and convalescent sera is indicative of an acute measles infection. Previously vaccinated individuals (secondary vaccine failure) may represent an exception in that rapid elevation of anti-measles IgG titre would be expected causing strong positive anti-measles IgG results in acute sera and the likely absence of a four-fold rise in IgG titre in the convalescent sera (36,37). The presence of measles-specific IgG antibodies, as determined using an enzyme immunoassay (EIA) predicts protective immunization, but it does not necessarily correlate with protective neutralising antibodies measured by the gold standard, the plaque reduction neutralization test—PRNT (38). The absence of detectable measles IgG using EIA may reflect the lower sensitivity of the EIA in comparison to a more sensitive assay, especially in young infants. The protective level of measles IgG has been estimated between 120 (39) and 200 mIU (40), but it is not precisely known.
SSPE diagnosis	Sub-acute sclerosing panencephalitis (SSPE) is a rare complication caused by persistent measles virus infection in the central nervous system. In the presence of the characteristic clinical, neurological and pathology signs, the diagnosis can be confirmed by detecting an increase of measles IgG titre in the cerebrospinal fluid (CSF) relative to the titre in serum.
Measles virus detection	The RT-PCR assay is the most reliable test for the definitive diagnosis of measles infection, but its sensitivity can be influenced by the following: • timing of the specimen collection • specimen integrity (rapid specimen processing) and storage conditions • prior vaccination history Measles virus isolation in culture is also a very specific test (when confirmed by immunofluorescence or RT-PCR), but it is less sensitive than RT-PCR and is heavily dependent on timely collection and specimen integrity.
Genotyping	Measles virus genotyping is needed to distinguish post-vaccine rash from wild type measles infection. Virus genotyping is useful for linking cases, linking outbreaks and tracking importations. PAHO requires genotype information for monitoring the efforts of elimination. It is advisable to genotype as many cases as possible of measles, ideally all the sporadic cases and representative cases of all the outbreaks. The National Microbiology Laboratory (NML) performs measles genotyping in Canada and will accept all suitable specimens. The genotyping test requires the same type of specimens as RT-PCR.
Interpretation of laboratory results	In order to properly interpret laboratory results and to assess the performance of measles diagnostic assays, both clinical and epidemiologic information need to be considered along with the laboratory information (e.g. prior vaccination history, travel history, timing of sample collection relative to onset of symptoms). Therefore, communication and information sharing between public health and the laboratory are essential. A positive RT-PCR result or positive IgM result in a patients with rash and with history of travel in a measles endemic area or with an epidemiological link to a confirmed case, are diagnostic for measles infection. Seroconversion or a fourfold increase of measles IgG in a patient with rash and no history of recent MMR vaccination is also diagnostic for measles. Negative results by RT-PCR and negative IgM-class antibody detection may not be sufficient to rule out measles infection in some cases, particularly if the specimen for PCR was collected later than 7 days after symptom onset. Serological results for previously vaccinated individuals (secondary vaccine failure) will likely not follow the paradigm associated with acute primary measles in unvaccinated individuals. Anti-measles IgM antibody response may be weak or not detectable, and a rapid elevation of anti-measles IgG titre would be expected causing strong positive anti-measles IgG results in acute sera and the likely absence of a four-fold rise in IgG titre in the convalescent sera (36,37). In these individuals, the timely collection of specimens for measles virus detection (RT-PCR) is recommended.