How to measure renal function in clinical practice

Traynor et al state that serum creatinine concentration, and therefore eGFR, may only be slightly affected by ingestion of meat.¹ In the data from the modification of diet in renal disease study that were used to generate the eGFR equations,² samples were taken from predominantly fasting subjects (AS Levey, personal communication). In clinical practice, however, samples for serum creatinine concentration and eGFR are generally used in situations where the patient's recent dietary intake is not considered.

We investigated the impact of meals on serum creatinine concentration and eGFR.³ Participants (n=32; median age 54.5, range 18-86) had blood samples taken before and after normal helpings of meat-containing meals supplied by our hospital canteen. Median serum creatinine concentration rose from 80.5 µmol/l before eating to 101.0 µmol/l 1-2 hours after eating (P<0.0001), and 99.0 µmol/l 3-4 hours after eating (P<0.0001). Furthermore, median eGFR fell from 84.0 ml/min/1.73m² preprandially to 59.5 ml/min/1.73m² 1-2 hours after eating (P<0.0001) and 64.0 ml/min/1.73m² 3-4 hours after eating (P<0.0001).

This led to apparent changes in staging of chronic kidney disease.⁴ In 12 of the 32 participants (six men aged 47-76; six women aged 36-84), the lowest eGFR in the postprandial period fell into a worse category than the preprandial eGFR. In 11 cases, chronic kidney disease staging was altered from better than stage 3 (which includes normal GFR, stage 1, and stage 2) to stage 3. In these cases, preprandial eGFRs ranged from 67 ml/min/1.73 m² to 97 ml/min/1.73 m² and the 60 ml/min/1.73 m² threshold was crossed. In the other case, staging changed from stage 3 to stage 4.

Our results suggest that the risk of misdiagnosis or incorrect staging of chronic kidney disease is high after a meal containing cooked meat. We recommend that national guidelines incorporate the advice that serum creatinine measurement, for the purpose of eGFR calculation and staging of chronic kidney disease, should be carried out when a patient has fasted or specifically avoided a cooked meat meal on the day of blood sampling.