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Vitamin D (25 hydroxyvitamin D [25(OH)D]) deficiency is common in patients with chronic kidney disease (CKD). Neither the relation of this deficiency to the decrease in glomerular filtration rate (GFR) nor the effects on CKD mineral and bone disorders (MBD) are clearly established.Cross-sectional analysis of baseline data from a prospective cohort, the NephroTest Study.1,026 adult patients with all-stage CKD not on dialysis therapy or receiving vitamin D supplementation.For part 1, measured GFR (mGFR) using (51)Cr-EDTA renal clearance; for part 2, 25(OH)D deficiency at15 ng/mL.For part 1, 25(OH)D deficiency and several circulating MBD markers; for part 2, circulating MBD markers.For part 1, the prevalence of 25(OH)D deficiency was associated inversely with mGFR, ranging from 28%-51% for mGFR ≥60-15 mL/min/1.73 m(2). It was higher in patients of African origin; those with obesity, diabetes, hypertension, macroalbuminuria, and hypoalbuminemia; and during winter. After adjusting for these factors, ORs for 25(OH)D deficiency increased from 1.4 (95% CI, 0.9-2.3) to 1.4 (95% CI, 0.9-2.1), 1.7 (95% CI, 1.1-2.7), and 1.9 (95% CI, 1.1-3.6) as mGFR decreased from 45-59 to 30-44, 15-29, and15 (reference, ≥60) mL/min/1.73 m(2) (P for trend = 0.02). For part 2, 25(OH)D deficiency was associated with higher age-, sex-, and mGFR-adjusted ORs of ionized calcium level1.10 mmol/L (2.6; 95% CI, 1.2-5.9), 1,25 dihydroxyvitamin D concentration16.7 pg/mL (1.8; 95% CI, 1.3-2.4), hyperparathyroidism (1.8; 95% CI, 1.3-2.4), and serum C-terminal cross-linked collagen type I telopeptides concentration1,000 pg/mL (1.6; 95% CI, 1.0-2.6). It was not associated with hyperphosphatemia (phosphate1.38 mmol/L).Cross-sectional analysis of the data prevents causal inferences.25(OH)D deficiency is related independently to impaired mGFR. Both mGFR decrease and 25(OH)D deficiency are associated with abnormal levels of circulating MBD biomarkers.

Background Serum cystatin C was proposed as a potential replacement for serum creatinine in glomerular filtration rate (GFR) estimation. We report the development and evaluation of GFR-estimating equations using serum cystatin C alone and serum cystatin C, serum creatinine, or both with demographic variables. Study Design Test of diagnostic accuracy. Setting & Participants Participants screened for 3 chronic kidney disease (CKD) studies in the United States (n = 2,980) and a clinical population in Paris, France (n = 438). Reference Test Measured GFR (mGFR). Index Test Estimated GFR using the 4 new equations based on serum cystatin C alone, serum cystatin C, serum creatinine, or both with age, sex, and race. New equations were developed by using linear regression with log GFR as the outcome in two thirds of data from US studies. Internal validation was performed in the remaining one third of data from US CKD studies; external validation was performed in the Paris study. Measurements GFR was measured by using urinary clearance of iodine-125–iothalamate in the US studies and chromium-51–EDTA in the Paris study. Serum cystatin C was measured by using Dade-Behring assay, standardized serum creatinine values were used. Results Mean mGFR, serum creatinine, and serum cystatin C values were 48 mL/min/1.73 m 2 (5th to 95th percentile, 15 to 95), 2.1 mg/dL, and 1.8 mg/L, respectively. For the new equations, coefficients for age, sex, and race were significant in the equation with serum cystatin C, but 2- to 4-fold smaller than in the equation with serum creatinine. Measures of performance in new equations were consistent across the development and internal and external validation data sets. Percentages of estimated GFR within 30% of mGFR for equations based on serum cystatin C alone, serum cystatin C, serum creatinine, or both levels with age, sex, and race were 81%, 83%, 85%, and 89%, respectively. The equation using serum cystatin C level alone yields estimates with small biases in age, sex, and race subgroups, which are improved in equations including these variables. Limitations Study population composed mainly of patients with CKD. Conclusions Serum cystatin C level alone provides GFR estimates that are nearly as accurate as serum creatinine level adjusted for age, sex, and race, thus providing an alternative GFR estimate that is not linked to muscle mass. An equation including serum cystatin C level in combination with serum creatinine level, age, sex, and race provides the most accurate estimates.