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Study Says Fasting Necessary for Creatinine Testing

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November 15, 2013

Although current standards for creatinine blood tests do not require that patients fast prior to sample collection, a new study published online in Diabetes Care recommends that the blood sample should be collected after fasting for 12 hours. The researchers found that eating cooked meat several hours prior to testing can lead to inaccurate interpretation of results.

Cooking converts a substance in meat called creatine to creatinine and, when consumed, temporarily increases the level of creatinine in the blood, possibly causing artificially high test results.

The main concern is that inaccurate creatinine test results could have a detrimental impact on the management of patients with chronic kidney disease (CKD). Healthy kidneys process and eliminate creatinine from the blood at a constant rate, so measuring creatinine levels gives a good indication of how well the kidneys are working. Levels in the blood rise when the kidneys are not functioning properly as seen in people with CKD.

Creatinine is also used to calculate the estimated glomerular filtration rate (eGFR), a measurement that helps health practitioners determine whether treatment or additional testing is needed for people with kidney problems. Incorrect creatinine and eGFR measurements could cause them to misclassify the stage of kidney disease to a more severe level than actually is present.

This is especially critical for diabetics, who are typically tested on a regular basis as a result of their high risk for kidney disease. High blood glucose levels that can occur with diabetes can damage the kidneys and prevent them from effectively filtering creatinine from the blood. Health care providers rely on creatinine test results and the eGFR to assess the degree of kidney damage in their diabetic patients.

The study included 80 participants divided into 5 groups; 16 were in the healthy group and 64 who had diabetes were divided into the remaining 4 groups based on their stage of CKD. Researchers tested the participants at two different sessions: fasting overnight, then eating a standard amount of meat (Angus burger), and in a separate session, substituting a non-meat product (vegetarian burger). Fasting blood samples were collected, with additional samples drawn again at 1, 2, and 4 hours after eating the morning meal. Creatinine levels were measured and eGFR calculated for each of the samples.

The researchers concluded that among all five participant groups, there were significant differences between fasting creatinine levels and those collected after consuming a meal of cooked meat. The differences were more marked in participants who had some degree of kidney dysfunction compared to healthy participants.

For example, at 2 hours after eating cooked meat, healthy participants had a median difference of 0.06 mg/dL (5 micromole/L) higher and those with moderately reduced kidney function had a median difference of 0.20 mg/dL (18 micromole/L) higher. [The normal reference range for blood creatinine is 0.9 - 1.3 mg/dL for adult men and 0.6 - 1.1 mg/dL for adult women.] There were no significant differences when non-meat meals were substituted.

Most notably, researchers found that 6 out of 16 participants with moderately reduced kidney function could have been misclassified as having more severe CKD than what they actually had based on non-fasting creatinine results. This could result in these patients undergoing unnecessary follow-up testing or treatment.

The researchers were not able to determine exactly when creatinine levels are highest after eating meat. A possible explanation for this is that people absorb creatinine from meat at different rates, causing blood creatinine to peak at varying times. They were able to demonstrate that the type of meal that was eaten the night before fasting (meat or non-meat) had no significant effect on creatinine levels in fasting samples.

According to the study's authors, eating meat prior to testing has a noticeable effect on blood creatinine levels. Based on this observation, they suggest the simple solution of measuring creatinine and eGFR from a fasting blood sample.

However, the researchers recognize that fasting is not so simple for some diabetics. Lead author, Dr. Sunil Nair from the University Hospital Aintree, Liverpool, United Kingdom, told Medscape Medical News that "from a clinical perspective, there will be a risk of hypoglycemia in insulin-treated patients [and] the recommendation could be modified to [merely] avoiding cooked meat prior to blood sampling to measure renal function."

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Tests: Creatinine, eGFR
Conditions: Kidney Disease

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used. To access online sources, copy and paste the URL into your browser.

Nair S, et al. Effect of a cooked meat meal on serum creatinine and estimated glomerular filtration rate in diabetes-related kidney disease. Published online before print September 23, 2013, doi: 10.2337/dc13-1770 Diabetes Care. Available online at through Accessed November 2013.

(October 2, 2013) Miriam E. Tucker. Eating Cooked Meat Can Distort CKD Stage in Diabetes. Medscape Medical News. Available online at through Accessed November 2013.

The Renal Association. CKD Stages. Available online at through Accessed November 2013.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 5th edition, St. Louis: Elsevier Saunders; 2011.