Measurement of immunoglobulin A in saliva by particle-enhanced nephelometric immunoassay: sample collection, limits of quantitation, precision, stability and reference range

BACKGROUND: Total immunoglobulin A in saliva (s-IgA) is normally assayed using an enzyme-linked immunosorbent assay. We have investigated methodological issues relating to the use of particle-enhanced nephelometric immunoassay (PENIA) to measure s-IgA in whole unstimulated saliva and determine its reference range. METHODS: Whole unstimulated resting saliva was collected to determine sample stability (temperature, time, effect of a protease inhibitor), limit of quantitation (LOQ), assay precision and analytical variation. The reference range for 134 healthy adults was determined. RESULTS: Linearity was excellent (4-10.3 mg L(-1), P < 0.001; R(2) = 0.997) and without significant bias (mean of -0.7%). The lowest intra- and inter-analytical coefficients of variation were 1.8% and 7.5% and LOQ was 1.4 mg L(-1). The concentration of s-IgA is stable at room temperature for up to 6 h, at 4 degrees C for 48 h, at -4 degrees C for two weeks and at -80 degrees C for up to 1.3 yr. There is no evidence that a protease inhibitor increases the stability or that repeated freeze-thawing cycles degrade sample quality. The reference ranges for s-IgA concentration, s-IgA secretion, s-IgA:albumin and s-IgA:osmolality were 15.9-414.5 mg L(-1), 7.2-234.9 microg min(-1), 0.4-19 and 0.6-8.9, respectively. CONCLUSION: Automated PENIA assay of s-IgA is precise and accurate. High stability of collected saliva samples and the ease and speed of the assay make this an ideal method for use in athletic and military training situations. The convenience of measuring albumin and IgA on the same analytical platform adds to the practicability of the test.