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We describe a fully automated particle-enhanced turbidimetric assay for cystatin C in undiluted serum and EDTA-plasma. The throughput is 90 samples per hour and urgent samples can be analyzed in 7 min. The assay range (0.4-14.1 mg/L) covers the concentration range in health and disease. The within- and between-run imprecision is 0.9% and 2.2%, respectively. Analytical recovery of additions of recombinant cystatin C averaged 98%. Rheumatoid factors (< or = 323,000 IU/L), bilirubin (< or = 150 mumol/L), hemoglobin (< or = 1.2 g/L), and triglycerides (< or = 8.5 mmol/L) do not interfere in the assay. In view of the superior (by ROC analysis) diagnostic accuracy of serum concentrations of cystatin C for reduced glomerular filtration rate (GFR) in comparison with creatinine, cystatin C seems an attractive alternative to creatinine for estimation of GFR.

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Assignment of the G3m(g) and (b) correlative amino acid residues was performed at the genomic level by direct sequencing of DNA from nine Caucasian individuals. Two oligonucleotide primers were used for subclass-specific enzymatic amplification of a DNA segment comprising a major portion of the second and third constant region domains (CH2 and CH3) of the human IgG3 heavy chain gene. Comparison of the sequences of amplified DNA from individuals serologically typed as homozygous for G3m(b) or G3m(g) or as heterozygous, G3m(b,g), revealed differences in the codons for the amino acid residues 291, 296, and 384. Proline, phenylalanine, and serine at these positions corresponded to G3m(b), and leucine, tyrosine, and asparagine to G3m(g). Heterozygotic individuals, typed G3m(b,g), displayed both the G3m(b) and G3m(g) codons at these three positions. The polymorphism at each of these three codons could be identified either as the appearance, or the loss, of recognition sites for the two restriction endonucleases, Nsp BII and Rsa I. This allowed the development of a polymerase chain reaction (PCR)-based assay permitting the distinction of G3mb and G3mg alleles by analyzing the electrophoretical mobility of the DNA fragments generated by digestion of the PCR-products with Nsp BII and Rsa I.