Comparison of acid and enzymatic methods for inulin dosage: analytical performances and impact on glomerular filtration rate evaluation.

Among issues susceptible to hamper a reliable measurement of inulin clearance, those regarding the dosage of inulin are largely neglected. We have compared the analytical performances of 2 commonly used methods of inulin dosage (one "acid" and one "enzymatic" method) and studied their potential impact on the glomerular filtration rate (GFR) value given by inulin clearance. Repeatability, uncertainty and the beta-expectation limits were evaluated from pre-determined serum and urine pools of inulin. Agreement between the two methods was analyzed from 99 inulin clearances performed in renal transplant patients. Impact of the method of dosage on GFR evaluation was simulated according to the respective beta-expectations limits of each method. Overall, intra-assay coefficient of variability and relative bias were inferior to 5% and 10% for both methods. Contrary to the acid method, analytical performance of the enzymatic method was not influenced by the presence of glucose. The relative difference in GFR values obtained with the two methods in transplant patients was -0.4 ± 10%. Simulations suggested that changes in inulin concentration attributable to analytical error could modify the value of GFR from -12% to +28%. In conclusion, while analytical performances are globally acceptable for both methods, they are not strictly equivalent. The impact on the determination of GFR, albeit limited, is not negligible and adds to other sources of inaccuracy. International standardization for the dosage of inulin is necessary.