With a few exceptions, most EIA kits for IgM testing used in the GMRLN perform well. With the quality assurance activities implemented throughout the network, confidence in the accuracy of laboratory classification provided by routine IgM testing is very high. However, there are limitations associated with IgM detection for case classification. As discussed below, ‘false negative’ IgM results can occur due to ‘early’ collection of specimens.
Note: A serum specimen is considered an ‘early’ specimen when it is collected ≤3 days from rash onset (suspect measles case) or ≤5 days after rash (suspect rubella case). The guidelines for the interpretation of results obtained by EIA for IgM detection includes a caveat, based on Helfand et al. [1], that a “false negative” may be obtained when serum is collected early. This refers to the possibility that the negative IgM result by EIA may not reflect the true disease status (a proportion of early serum specimens from true cases will have a negative IgM result). A negative IgM result under such circumstances is not technically a false-negative result, but represents a valid result as measured by the EIA since the patient has either not yet produced an IgM response or the level of virus-specific IgM is below the detection threshold for the EIA.
A false positive IgM result may be obtained when serum is tested for measles or rubella IgM from patients with rash illnesses due to other causes. A false positive IgM result, or nonspecific positive result, may also be due to components within the serum specimen. The possible causes for a nonspecific reaction in the IgM assay are discussed in section 4.2.2. In addition, there are guidelines that should be considered if a positive IgM result is obtained from a suspected case that has been recently immunized with the measles monovalent or the MR/MMR vaccine (section 4.3).
Laboratory test results obtained by IgM EIAs should be reported promptly with appropriate accompanying information as described in Chapter 11. Data management and reporting of laboratory results. Additional serologic testing for virus-specific IgG may aid in case classification when the IgM result is equivocal or to provide additional results to resolve an IgM positive result that is doubtful or unexpected due to a non-classic clinical presentation and/or the epidemiologic investigation fails to identify a source of infection (section 4.6). If equivocal results cannot be resolved, the result is reported as equivocal. Additional testing is not indicated in settings where disease is endemic or during outbreaks and the cases are classified as confirmed. See Figure 4.1, Flowchart for case classification of measles in non-elimination settings. The detection of virus-specific RNA by RT-PCR can be helpful to support a doubtful positive or an equivocal IgM result. The application of RT-PCR for case classification is discussed in Chapter 6. Detection of viral RNA by RT-PCR for the confirmation of measles and rubella infection.
Note: In areas that are close to elimination, or those that have achieved elimination, the flowchart for case confirmation and for discarding cases has been modified to include testing strategies that can provide enhanced laboratory support for accurate case classification. A flowchart (figure 8) for measles and rubella cases classification in near or post elimination settings is included in Chapter 8. Laboratory testing in support of measles and rubella surveillance in elimination settings.
4.2.1 Timing of serum collection
An adequate serum specimen for surveillance purposes is one that is collected within 28 days from onset of rash. As discussed above, although most commercial IgM EIAs are highly sensitive, a negative IgM result may be obtained from a proportion of measles cases if serum was collected ≤3 days (≤5 days for rubella cases) after rash onset. Particularly in elimination settings, a second serum specimen is recommended if a negative IgM result is obtained from serum collected during the timeframe that is considered early for IgM detection by EIA. Despite the possibility that a false negative result may be obtained, serum should be collected at first contact with suspected case. At the same time, collection of a virologic specimen is recommended for molecular surveillance and for RT-PCR testing, which can improve the ability to rapidly confirm cases. Refer to Chapter 3. Clinical specimens for the laboratory diagnosis and molecular epidemiology of measles, rubella, and CRS for best practices for collection of serum specimens.
4.2.2 Cross-reactions and interference: false positive results for IgM
Many pathogens and medical conditions can produce a rash and other symptoms that mimic measles or rubella infection. Many of these etiologic agents or conditions can also cause a non-specific or false-positive IgM result in measles or rubella IgM assays. Medical conditions in which high levels of rheumatoid factor (RF) are present can cause interference and generate false positive IgM results.
Numerous investigations have been conducted to determine the etiologic agents responsible for rash illness from non-measles/non-rubella cases. Parvovirus B19 was identified as the cause of rash among a high proportion of cases of rash illness not due to measles or rubella [2-5]. Enterovirus and adenovirus were also commonly identified. Among younger children with rash, HHV-6 was another frequent etiologic agent identified [2-5].
Several of the GMRLN laboratories participate in national and international surveillance programmes for other infectious diseases including laboratory support for confirmation of many viral diseases associated with rash and fever, particularly those that are mosquito-borne diseases that occur as epidemics. Approximately one-third of primary dengue virus infections can present with rash and fever, and when a dengue epidemic occurs, the majority of rash and fever cases that are reported are caused by dengue. Chikungunya virus and zika virus are other mosquito-borne diseases of public health importance that can cause rash and fever. If IgM testing is conducted on these cases to rule out the possibility of measles or rubella, the test results may be positive when the rash and fever is due to one of these mosquito-borne diseases. The viremia from these agents can cause non-specific reactions or formation of immune complexes that can produce a false positive IgM result in measles or rubella IgM assays.