As the number of patients seeking bariatric surgery increases, it is important to assess the risks and benefits associated with these procedures.

Bariatric surgery is employed to aid morbidly obese patients in obtaining a healthy body weight. Although this surgery may provide positive outcomes, many factors associated with obesity increase the risks for complications in surgery. 

A recent study by Turgeon et al (J Am Soc Nephrol 2012;23:885-894) compared chronic kidney disease (CKD) stage and risk for complications from bariatric surgery.

The investigators evaluated 27,736 patients who underwent bariatric surgery between 2006 and 2008. Of these, 20,806 (75%) patients had CKD stage 1, 5,011 (18.07%) had CKD stage 2, 1,734 (6.25%) had CKD stage 3, 94 (0.34%) had CKD stage 4, and 91 (0.33%) had CKD stage 5. Higher CKD stage correlated significantly with increased hospital stay and higher rate of return to the operating room. With respect to postoperative events, as CKD stage increased, so did the risk for complications.

Diabetes and hypertension were the primary confounders. Multivariate analyses that controlled for body mass index, anesthesiology classification, and type of procedure found that CKD stage was still an independent predictor of post-operative complications. Each increment in CKD stage was associated with a 17.8% increased risk for complications in adjusting analyses.

Renal function and bariatric surgery

Another recent study took advantage of the dramatic weight changes that can accompany bariatric surgery to review the changes in estimated glomerular filtration rate (eGFR) before and after surgery (Int Urol Nephrol 2012; published online ahead of print). The increased body surface area (BSA) of obese individuals can confound eGFR. A total of 220 patients who underwent bariatric surgery were followed for six months.

Exclusion criteria included loss to follow-up, chronic nephrotoxic medication use, and underlying chronic illness/malignancy. The following equations were used to determine eGFR: Modification of Diet in Renal Disease (MDRD) study equation, CKD-Epi, CKD-Epi with BSA-adjusted versions, Cockcroft-Gault (CG), and Cockgroft-Gault lean body weight-adjustment formula (CG-LBW).

Serum creatinine decreased significantly from 63 to 58 mg/dL, BMI decreased significantly from 47 to 36.12 kg/m², and BSA decreased significantly from 2.2 to 1.8 m². When analyzing all subjects, these changes led to significantly decreased eGFR as calculated using the CG, CG-LBW, adjusted CKD-Epi, and adjusted MDRD equations. Some of these equations indicated a shift from hyperfiltration to normal ranges, but there was high variability in readings. LBW appeared to be the best control when accounting for obese populations.

In a subgroup of patients who had a preoperative eGFR of 60-90, eGFR calculated using the CG-LBW equation increased significantly after surgery, whereas the eGFR calculated using the adjusted CKD-Epi equation increased significantly until corrected with BSA.

Serum creatinine decreased significantly in both groups; this was most likely related to a reduction in LBW and a reduction in protein intake. The confounding effect of LBW may indicate that there were no permanent changes in renal function, and further follow-up would be necessary to see if this change in eGFR continued.

Assessing the risk of complications

In the study by Turgeon et al, eGFR was calculated using the CKD-Epi formula. It is important to note that the data from this study still proved to be significant even with the possible confounders such as BSA and LBW. Thus, the results of this study indicate that the CKD-Epi creatinine formula can still be useful as a guide when assessing a patient's risk of complications. CKD stage 4 only showed a 2.3% increase in risk for complications after multivariate controls were applied.

These studies must be considered carefully because it has previously been shown that increased BMI offers a protective effect in the latter stages of CKD (Nephrol Dial Transplant 2009;24:2421-2428). The sample size of CKD stages 4 and 5 were much smaller than the other cohorts, and further studies would be beneficial to see if a protective effect of BMI was influencing the rate of complications in the CKD stage 4 group. The CKD stage 5 group suffered a much more dramatic increase in risk for complications. This group included those on dialysis, and thus BMI may have offered less of a protective effect. 

The appropriateness of bariatric surgery in CKD populations needs further research. At this time, it appears that as eGFR declines, bariatric surgery may place patients at greater risk for complications. In addition, it is important to note that when assessing eGFR in this context, equations that account for LBW appear to offer the most accuracy.

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