Two types of samples have been evaluated by specialized and reference laboratories in the GMRLN for detection of rubella- or measles-specific IgM as an alternative to testing serum. Dried blood spots (DBS) can be prepared from capillary or venous blood and may provide a viable option in areas where a cold chain and/or collection of venous blood is not feasible. Collection of oral fluid (OF) is minimally invasive and may not require refrigeration under certain conditions [7]. However, these alternative types of samples require that proper techniques for collection and processing are followed. The best practices for the collection, optimal timing, and preparation of DBS and OF are presented in chapter 3.
The use of oral fluid (OF) specimens for IgM antibody testing requires that the EIA kit has been validated for OF, since the OF sample is typically analysed without dilution. A sufficient amount of total IgG should be present in the sample as an indication that the specimen would potentially contain measurable IgM [8, 9]. A capture EIA is used for IgM detection in OF. Details regarding modification of the protocol for the EIA for testing OF is provided in Annex 4.1.
In an evaluation of these alternative specimens, the sensitivity and specificity for measles- and rubella-specific IgM were comparable to measurement of IgM using serum specimens [7]. However, the early collection of the specimens is reflected in the concordance of IgM results obtained from both OF and serum. In a study of suspected rubella cases, detection of IgM from serum was somewhat higher in the first 4 days after onset of rash than the corresponding results for OF samples [10]. However, if the equivocal results for IgM from OF were considered as positive results, the concordance was much higher. The concordance of results for IgM detection from OF and serum improved considerably when specimens were collected ≥4 days after rash onset. As previously noted, the use of RT-PCR for RNA detection confirmed more rubella cases than IgM detection from serum among specimens that were collected within 2 days of rash onset.
The concordance observed for IgM detection using paired DBS and serum specimens may vary in similar ways as that noted for OF and serum. A study was conducted in Uganda to compare results for measles IgM detection using DBS and serum collected from hospitalised children with measles [11]. The concordance observed between IgM results from DBS (eluate) and serum when the specimens were collected on day 0-6 after rash onset was 95.6% (253 sample pairs). However, of 260 paired samples that were collected from week 2 to 5 after rash onset, the concordance was 100%. [11]. In a study of rubella IgM detection comparing DBS and serum, the concordance of results improved when equivocal results for IgM from DBS were considered as positives. In addition, the collection of samples at days 4-28 increased concordance from 76% to 88%. [12].
Eluted fluid from dried blood spots (DBS) can be tested using a commercially available IgM indirect EIA or a capture EIA which has been validated for the purpose of testing DBS. Methods for the elution of serum specimens from DBS have been published [13], and recommended protocols are provided in Annex 3.2.