Creatinine reference values in ELBW infants: impact of quantification by Jaffe or enzymatic method.

OBJECTIVE: Serum creatinine (Scr) reflects to a certain extent glomerular filtration rate in neonates, but postnatal observations also depends on the technique used to quantify Scr (Jaffe colorimetry or enzymatic quantification). METHODS: In an attempt to quantify differences between these techniques, we compared postnatal Scr trends in two consecutive cohorts of extremely low birth-weight (ELBW) neonates before and following a switch from uncompensated Jaffe to enzymatic Scr quantification. Postnatal Scr (Days 1, 2, 3, 4, 5, 6, 7, 8, 9, 14, 21, 28, and 42) in 151 ELBW neonates (uncompensated Jaffe) was compared to 116 more recently admitted ELBW neonates (enzymatic). RESULTS: Although clinical characteristics were similar between both cohorts, median postnatal Jaffe Scr values were significantly higher compared to enzymatic quantification (all days, at least p < 0.001) throughout postnatal life. While both cohorts displayed a similar trend with an initial increase with a Scr peak on Days 3 and 4 and a subsequent decrease, the difference in within-day median values fluctuated between 11 and 24 mmol.L(-1). There is neither fixed nor relative difference in Scr between both techniques. CONCLUSIONS: When using Scr to estimate renal function in ELBW neonates, clinicians should in addition to the postnatal changes and other covariates of renal function, also consider the technique applied. We provide reference values and comparison between both techniques.

Creatinaemia at birth is equal to maternal creatinaemia at delivery: does this paradigm still hold?

OBJECTIVE: The paradigm that creatinaemia at birth is equal to maternal creatinaemia may also depend upon the quantification technique applied. Paired maternal-neonatal creatinaemia samples in whom Jaffe in both or compensated Jaffe (maternal) and enzymatic quantification (neonate) were applied. METHODS: Extreme low birth weight infants in two time intervals were included when paired maternal-neonatal creatinaemia samples were available. In cohort 1 (2000-2005), creatinaemia (mothers and neonates) was based on Jaffe assay. In cohort 2 (2007-2010), maternal creatinaemia was based on compensated Jaffe. In neonates, an enzymatic technique was applied. Unpaired Mann Whitney U, paired Wilcoxon and Bland-Altman were used. RESULTS: Based on 80 and 52 paired creatinaemia samples, there was no significant difference between maternal (0.80, 0.41-1.6 mg.dl(-1)) and neonatal creatinaemia (0.78, 0.31-1.46 mg.dl(-1)) in cohort 1 while a significant difference (p < 0.001) between maternal (0.6, 0.29-2.24 mg.dl(-1)) and neonatal creatinaemia (0.67, 0.4-2.2 mg.dl(-1)) was observed for cohort 2. Using Bland-Altman, the fit was perfect for cohort 1 (mean diff -0.02 mg.dl(-1)), but not for cohort 2 (-0.08 mg.dl(-1)). CONCLUSIONS: The quantification method affects the paradigm that creatinaemia at birth is similar to maternal creatinaemia. Maternal and neonatal creatinaemia values depend on the method used. Consequently, method-specific reference values are needed.