Evaluation and clinical significance of latex immunoassay (LIA) of insulin using the routine biochemical autoanalyzer

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Fasting and postprandial determination of insulin responses is a good marker for the evaluation of hypo and hyper-insulinemia and insulin resistance. Latex immunoassay (LIA) of insulin using routine biochemical autoanalyzer is thus highly useful and important. Serum samples were mixed with two types of monoclonal antibody against human insulin, which were conjugated to latex beads, and absorbance changes at 604/804 nm between 245 and 503 seconds were measured automatically. The intra-assay CV was 2% between 7 and 100μU/ml insulin and the minimum assay limit was 2μU/ml. No cross reaction was found with proinsulin. Hemoglobin, bilirubin, and rheumatic factor did not interfere the insulin measurement. Frozen serum samples at the low (7μU/ml) and medium (30μU/ml ) range of insulin were quite stable for 2 months, while at 100μU/ml, gradual reduction was noted at 1 week (5%) and 2 weeks (9%) and especially after 1 month. Therefore serum samples for LIA should be stored at 40° and analyses should be performed within a week.<BR>Time used to measure 20 samples was 12 min by LIA, while 39 min by EIA. Samples exceeding 100μU/ml was necessary to be diluted with saline.<BR>A good correlation between LIA and EIA (r=0.98) or RIA (r=0.99) was found during cookie tests in 25 healthy subjects with significant regression. LIA is thus simple, and insulin can be measured in the same machine as glucose or cholesterol.