Kidney stones are heterogeneous but often grouped together. The potential effects of patient demographics and calendar month (season) on stone composition are not widely appreciated. The first stone submitted by patients for analysis to the Mayo Clinic Metals Laboratory during 2010 was studied (n=43,545). Stones were classified in the following order: any struvite, any cystine, any uric acid, any brushite, majority (≥50%) calcium oxalate, or majority (≥50%) hydroxyapatite. Calcium oxalate (67%) was the most common followed by hydroxyapatite (16%), uric acid (8%), struvite (3%), brushite (0.9%), and cystine (0.35%). Men accounted for more stone submissions (58%) than women. However, women submitted more stones than men between the ages of 10-19 (63%) and 20-29 (62%) years. Women submitted the majority of hydroxyapatite (65%) and struvite (65%) stones, whereas men submitted the majority of calcium oxalate (64%) and uric acid (72%) stones (P<0.001). Although calcium oxalate stones were the most common type of stone overall, hydroxyapatite stones were the second most common before age 55 years, whereas uric acid stones were the second most common after age 55 years. More calcium oxalate and uric acid stones were submitted in the summer months (July and August; P<0.001), whereas the season did not influence other stone types. It is well known that calcium oxalate stones are the most common stone type. However, age and sex have a marked influence on the type of stone formed. The higher number of stones submitted by women compared with men between the ages of 10 and 29 years old and the change in composition among the elderly favoring uric acid have not been widely appreciated. These data also suggest increases in stone risk during the summer, although this is restricted to calcium oxalate and uric acid stones.

Recognizing that race is a social and not a biological construct, healthcare professionals and the public have called for removal of race in clinical algorithms. In response, the National Kidney Foundation and the American Society of Nephrology created the Task Force on Reassessing the Inclusion of Race in Diagnosing Kidney Diseases to examine the issue and provide recommendations. The final report from the Task Force recommends calculating estimated glomerular filtration rate (eGFR) without a race coefficient using the recently published CKD-EPI 2021 creatinine (cr) and creatinine-cystatin C (cr-cys) equations. The Task Force recommends immediately replacing older eGFR_cr equations (MDRD Study and CKD-EPI 2009) with the new CKD-EPI 2021 equation. In a 2019 survey by the College of American Pathologists, 23% of 6200 laboratories reporting eGFR_cr used an incorrect equation that is not suitable for use with standardized creatinine measurements, 34% used the CKD-EPI 2009 equation and 43% used the MDRD Study 2006 equation re-expressed for standardized creatinine measurement. Rapid transition to using the CKD-EPI 2021 equation is an opportunity for laboratories to standardize to a single equation to eliminate differences in eGFR_cr due to different equations used by different laboratories, and to report eGFR without use of race. We provide guidance to laboratories for implementing the CKD-EPI 2021 equations for both eGFR_cr and eGFR_cr-cys.

Roux-en-Y bypass surgery is the most common bariatric procedure currently performed in the United States for medically complicated obesity. Although this leads to a marked and sustained weight loss, we have identified an increasing number of patients with episodes of nephrolithiasis afterwards. We describe a case series of 60 patients seen at Mayo Clinic-Rochester that developed nephrolithiasis after Roux-en-Y gastric bypass (RYGB), including a subset of 31 patients who had undergone metabolic evaluation in the Mayo Stone Clinic. The mean body mass index of the patients before procedure was 57 kg/m² with a mean decrease of 20 kg/m² at the time of the stone event, which averaged 2.2 years post-procedure. When analyzed, calcium oxalate stones were found in 19 and mixed calcium oxalate/uric acid stones in two patients. Hyperoxaluria was a prevalent factor even in patients without a prior history of nephrolithiasis, and usually presented more than 6 months after the procedure. Calcium oxalate supersaturation, however, was equally high in patients less than 6 months post-procedure due to lower urine volumes. In a small random sampling of patients undergoing this bypass procedure, hyperoxaluria was rare preoperatively but common 12 months after surgery. We conclude that hyperoxaluria is a potential complicating factor of RYGB surgery manifested as a risk for calcium oxalate stones.

Summary: Background: The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation was developed using both CKD and non-CKD patients to potentially replace the Modification of Diet in Renal Disease (MDRD) equation that was derived with only CKD patients. The objective of our study was to compare the accuracy of the MDRD and CKD-EPI equations for estimating GFR in a large group of patients having GFR measurements for diverse clinical indications. Design, setting, participants, and measurements: A cross-sectional study was conducted of patients who underwent renal function assessment for clinical purposes by simultaneous measurements of serum creatinine and estimation of GFR using the MDRD and CKD-EPI equations and renal clearance of iothalamate (n = 5238). Results: Bias compared with measured GFR (mGFR) varied for each equation depending on clinical presentation. The CKD-EPI equation demonstrated less bias than the MDRD equation in potential kidney donors (−8% versus −18%) and postnephrectomy donors (−7% versus −15%). However, the CKD-EPI equation was slightly more biased than the MDRD equation in native CKD patients (6% versus 3%), kidney recipients (8% versus 1%), and other organ recipients (9% versus 3%). Among potential kidney donors, the CKD-EPI equation had higher specificity than the MDRD equation for detecting an mGFR <60 ml/min per 1.73 m² (98% versus 94%) but lower sensitivity (50% versus 70%). Conclusions: Clinical presentation influences the estimation of GFR from serum creatinine, and neither the CKD-EPI nor MDRD equation account for this. Use of the CKD-EPI equation misclassifies fewer low-risk patients as having reduced mGFR, although it is also less sensitive for detecting mGFR below specific threshold values used to define CKD stages.