Effect of beef ingestion by humans on plasma concentrations of creatinine, urea, and cystatin C.

PURPOSE: The effect of eating meat on serum concentrations of creatinine has varied among previous reports, with some finding no effect and others finding 50-100% increases, which appears related to how the beef is cooked. For other analytes related to kidney function, urea is well known to increase following a protein meal, and the effect of eating meat on cystatin C concentrations has been studied once. METHODS: We had 32 participants eat a measured amount of cooked beef (5-6 or 10-12 oz; 142-170 or 284-340 g) and collected blood for measurements at 1 h before and immediately before eating beef, then at 1, 2, and 4 h after eating the beef. We measured creatinine using both alkaline picrate and enzymatic methods, cystatin C using a nephelometric immunoassay, and urea using an enzymatic method. RESULTS: For creatinine, both the picrate and enzymatic methods showed similar responses, with a peak average increases of 5.9 µmol/L (0.07 mg/dL) and 4.6 µmol/L (0.05 mg/dL), respectively, at 2 h. Cystatin C had a very slightly maximal decrease of -0.037 mg/L at 2 h. Urea had the largest change, increasing by 0.30 and 0.77 mmol/L at 2 and 4 h respectively. CONCLUSIONS: Healthy individuals were found to have minor increases in serum creatinine (~5 µmol/L) following the ingestion of 5/6 or 10/12 oz of fried beef. Cystatin C appears to decrease very slightly in some people after beef ingestion, possibly due either to circadian variation or to a hormonal effect of eating. We conclude that ingesting these amounts of fried beef has a small effect on plasma creatinine concentrations. Although these increases would likely not affect the diagnosis of a kidney impairment in this population or in those with kidney disease, eating meat before collecting blood for creatinine measurement should be avoided.

The analytical change in plasma creatinine that constitutes a biologic/physiologic change.

PURPOSE: Accurate and precise measurements of creatinine are necessary to evaluate changes in kidney function related to a decreased glomerular filtration rate (GFR). When serial measurements of creatinine are monitored in an individual, it is useful to know what magnitude of an analytical change in creatinine indicates a true physiologic/biologic change in plasma creatinine that might warrant clinical intervention. METHODS: We compared results between three different methods for creatinine using large chemistry analyzers, two based on alkaline picrate (AP1 and AP2), and one based on dry-slide enzymatic conversion (ENZ). On each of three different segments or days of the study spaced 1-2months apart, we selected 10 different plasma samples having creatinine concentrations ranging from about 0.5mg/dL to 4.5mg/dL (44 to 400µmol/L). Each sample was analyzed in triplicate on each of two same-model analyzers at each institution, then from this data we determined the precision of each model of analyzer. The within-instrument precision of each analyzer was evaluated from the differences between the triplicate results on each sample by each analyzer (mean and SD of the differences). The between-instrument precision was evaluated as the differences between results on the same sample (1, 2, 3, etc.) analyzed on different analyzers of the same model (A and B). This between-analyzer precision data was used to determine both the range and mean±2SD of the differences that could be used to indicate that greater changes in creatinine concentrations would represent a biologic change. RESULTS: The within-instrument precision was best for the ENZ method in comparison to the two alkaline picrate rate methods. The between-instrument precision of the 90 consecutive measurements (30 samples×triplicate analyses) between the same-model analyzers were (mean and SD of differences in mg/dL): -0.018 and 0.029 (ENZ); 0.016 and 0.11 (AP1), and -0.058 and 0.071 (AP2). CONCLUSIONS: While all three of the creatinine methods studied had good precision, the ENZ method had the best precision, such that a change of 0.07mg/dL (6µmol/L) in serial creatinine concentrations up to 1.5mg/dL on a patient could indicate a biologic change had occurred. For the alkaline picrate methods, a measured change of creatinine of 0.23mg/dL for AP1 or 0.11mg/dL for AP2 would indicate that a physiologic change in serum/plasma creatinine has occurred. While a definite biologic change may simply represent daily variations, detecting a biologic change in creatinine more rapidly could impact the ability of creatinine to detect early and clinically significant changes in renal function.