Reliability of Plasma Creatinine Measurement in Infants and Children

Abstract

Plasma creatinine (Pcr, mg/dl) is often used to estimate glomerular filtration rate (GFR) in children. To establish whether the clinician can rely on the commonly used methods for measuring Pcr, the authors analyzed data from their own modified Technicon Autoanalyzer reference method and compared them to those obtained simultaneously from a Beckman Astra 8 kinetic Jaffe technique or a Technicon continuous flow Jaffe endpoint SMAC method. The SMAC method consistently overestimated Pcr by 0.1 mg/dl, whereas the kinetic method resulted in a large spread around the reference values. Neither laboratory gave consistent results for Pcr below 0.55, the normal range for infants and young children. The SMAC technique tended to underestimate GFR by 20 to 30 percent, whereas the kinetic method resulted in a great deal of scatter (only 37% of the measurements fell within ±25% of the values for GFR obtained by the reference method). The results suggest that the subtraction of 0.1 mg/dl from the Pcr measured on the SMAC system would give a value similar to that obtained with the reference method. This correction would permit the use of an estimate of GFR from kL/Pcr, where L is body length in cm and k is a constant (equalling 0.45 in term infants, 0.55 in children, and 0.7 in adolescent boys). The kinetic method requires repetitive determinations of Pcr before any firm conclusions can be drawn about GFR because of the scatter. The reliability of the clinician's laboratory can be tested by sending half the plasma to the laboratory on one day and the other half the next; the values should usually differ by <0.1 mg/dl or <15 percent, otherwise another laboratory should be consulted. It is suggested that both the kinetic and SMAC methods may lead to inaccuracies in the measurement of Pcr and therefore in the estimation of GFR in children from kL/Pcr.