Falsely increased values for rabbit immunoglobulin-based nephelometric immunoassays attributable to human anti-rabbit antibodies.

;25% false negatives. By contrast, plasma cystatin C better reflected GFR with the strongest correlation with Cr-labeled EDTA clearance and no false negatives. In a group of adult transplant recipients at steady-state renal function (average time since transplantation, 6 years), cystatin C was shown to have a better diagnostic value than serum creatinine for a GFR cutoff set at 60 mL/min (3 ). Despite a similar diagnostic value, 24-h creatinine clearance overestimated GFR by .20% in two-third of patients (3 ). The underestimation of GFR by plasma cystatin C we report here might be attributable to an inappropriate cutoff for cystatin C (too low), renal function not at steady state, or other unknown factors. Underestimation of GFR by serum cystatin C recently has been reported in pediatric transplantation (4 ). Serum cystatin C concentrations were 25% higher (range, 19–31%) in transplanted children vs non-transplanted children having the same GFR determined by inulin clearance (4 ). Interference with the cystatin C assay, tubular back-leakage of intact cystatin C into the circulation, or impaired filtration of antibody-cystatin C complexes have been suggested without definitive proof (4 ). Despite the underestimation of GFR, plasma cystatin C appears superior to creatinine and 24-h creatinine clearance for evaluation of GFR in the postoperative follow-up of adult kidney transplant recipients. Moreover, GFR can be rapidly estimated from the reciprocal of the plasma cystatin C concentration, using a simple formula independent of age, body surface, and sex of the recipient.