Renal function estimates may be inaccurate after bariatric surgery

By Rob Goodier

NEW YORK (Reuters Health) - Glomerular filtration rate may not accurately estimate renal function in patients who lose weight after bariatric surgery, a new study suggests.

In 11 women studied, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation based on serum creatinine levels underestimated the GFR at baseline and then overestimated it at six and 12 months as the patients rapidly lost weight.

"Because of the large change in weight after surgery, serum creatinine, the most common way kidney function is monitored, is no longer accurate. If knowledge of kidney function is important, other tests (such as cystatin C) could be considered," said lead author Dr. John Lieske, of the Mayo Clinic in Rochester, Minnesota, in email to Reuters Health.

The patients were morbidly obese at the start of the study with an average BMI of 46 kg/m2. The average fell to 33 kg/m2 after six months and then into the normal range at 28 kg/m2 after 12 months.

The researchers used renal iothalamate clearance to measure GFR.

As reported online July 29 in the American Journal of Kidney Diseases, they found that measured GFR, corrected for body surface area, decreased after Roux-en-Y gastric bypass by 10 mL/min/1.73 m2. Meanwhile, estimated GFR corrected for body-surface-area increased by 6 mL/min/1.73m2.

The change in renal function caught by measured GFR did not register in the estimate because of a decrease in creatinine generation, the researchers wrote in a research letter.

The fact that the study included only women does not imply that the findings might not also apply to men, Dr. Lieske says.

He offers one more piece of advice: "If serum creatinine stays the same or goes up slightly after surgery, it is important to realize that the patient may have lost kidney function. Investigation into possible causes may be needed."

Dr. Jonathan Carter, a bariatric surgeon at the University of California San Francisco, who was not involved in the research, said it's not surprising that the CKD-EPI equation is inaccurate in these patients.

He told Reuters Health, "The CKD-EPI equation was derived from a large cohort of patients and itself is limited in its accuracy; the equation estimates GFR to within 30% of true GFR in only 4 out of 5 patients. It is no surprise that when one takes a small subset (obese women), that the accuracy of CKD-EPI suffers more."

SOURCE: http://bit.ly/1A12D5E

Am J Kidney Dis 2014.

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