Analytical performance evaluation of a particle-enhanced turbidimetric cystatin C assay on the Roche COBAS 6000 analyzer.

OBJECTIVES: Cystatin C is a low molecular protein that has been proposed to estimate the glomerular filtration rate. Here we investigated the performance of the Roche cystatin C assay on the COBAS 6000 analyzer. DESIGN AND METHODS: We studied the imprecision, recovery, limit of detection and quantification, linearity and interferences. For method comparison, split sample aliquots were assayed using the described method and a Siemens cystatin C assay. RESULTS: The assay displayed a low total imprecision and a good linearity over the entire range tested. Bilirubin and triglycerides did not interfere with the assay, and only a haemoglobin concentration higher than 6g/dl interfered with the assay. The assay agreed well with the Siemens assay. CONCLUSION: The Roche cystatin C assay is an acceptable method for determining the cystatin C and the glomerular filtration rate estimate. On a COBAS 6000, the assay improves and simplifies the laboratory's workload.

Diagnosing insulin resistance by simple quantitative methods in subjects with normal glucose metabolism.

OBJECTIVE: To identify a reliable yet simple indirect method for detection of insulin resistance (IR). RESEARCH DESIGN AND METHODS: A total of 65 subjects (44 men and 21 women aged 30-60 years) were selected by a simple random sampling method. Inclusion criteria were voluntary participation from staff and hospital personnel, absence of abnormal glucose tolerance, and normal results of lipid profile and basic blood chemistry. A blood sample was taken after a 12-h overnight fast to determine plasma lipid, glucose, and insulin levels. An intravenous glucose tolerance test with administration of insulin after 20 min and extraction of multiple blood samples for glucose and insulin measurements and calculation of the minimal model approximation of the metabolism of glucose (MMAMG) S(i) value were performed. Three indirect indexes used to predict insulin sensitivity or IR were calculated, and metabolic syndrome was diagnosed using the Adult Treatment Panel III (ATP III) criteria. All results were correlated with those of the MMAMG. RESULTS: The 75th percentile value as the cutoff point to define IR corresponded with a fasting plasma glucose level of 12 mU/l, a homeostasis model assessment of 2.6, a 25th percentile for S(i) value of 21, and QUICKI (quantitative insulin sensitivity check index) and McAuley indexes of 0.33 and 5.8, respectively. The S(i) index correlated (P < 0.001) with all the indirect indexes and parameters of the metabolic syndrome. CONCLUSIONS: When compared with the S(i) index, the most sensitive and specific indirect method was the score proposed by McAuley et al. (specificity 0.91, sensitivity 0.75, 9.2 probability ratio of a positive test), followed by the existence of metabolic syndrome (specificity 0.91, sensitivity 0.66, 7.8 probability ratio of a positive test).