Estimation of GFR in South Asians: a study from the general population in Pakistan

Abstract

Background: South Asians are at high risk for chronic kidney disease. However, unlike those in the United States and United Kingdom, laboratories in South Asian countries do not routinely report estimated glomerular filtration rate (eGFR) when serum creatinine is measured. The objectives of the study were to: (1) evaluate the performance of existing GFR estimating equations in South Asians, and (2) modify the existing equations or develop a new equation for use in this population.

Study design: Cross-sectional population-based study.

Setting & participants: 581 participants 40 years or older were enrolled from 10 randomly selected communities and renal clinics in Karachi.

Predictors: eGFR, age, sex, serum creatinine level.

Outcomes: Bias (the median difference between measured GFR [mGFR] and eGFR), precision (the IQR of the difference), accuracy (P30; percentage of participants with eGFR within 30% of mGFR), and the root mean squared error reported as cross-validated estimates along with bootstrapped 95% CIs based on 1,000 replications.

Results: The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) creatinine equation performed better than the MDRD (Modification of Diet in Renal Disease) Study equation in terms of greater accuracy at P30 (76.1% [95% CI, 72.7%-79.5%] vs 68.0% [95% CI, 64.3%-71.7%]; P < 0.001) and improved precision (IQR, 22.6 [95% CI, 19.9-25.3] vs 28.6 [95% CI, 25.8-31.5] mL/min/1.73 m(2); P < 0.001). However, both equations overestimated mGFR. Applying modification factors for slope and intercept to the CKD-EPI equation to create a CKD-EPI Pakistan equation (such that eGFRCKD-EPI(PK) = 0.686 × eGFRCKD-EPI(1.059)) in order to eliminate bias improved accuracy (P30, 81.6% [95% CI, 78.4%-84.8%]; P < 0.001) comparably to new estimating equations developed using creatinine level and additional variables.

Limitations: Lack of external validation data set and few participants with low GFR.

Conclusions: The CKD-EPI creatinine equation is more accurate and precise than the MDRD Study equation in estimating GFR in a South Asian population in Karachi. The CKD-EPI Pakistan equation further improves the performance of the CKD-EPI equation in South Asians and could be used for eGFR reporting.

Keywords: CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) Pakistan; Glomerular filtration rate (GFR); South Asians; estimating equations; renal function.

Figure 1

Bias (measured glomerular filtration rate [GFR] − estimated GFR) by levels of estimated GFR. Solid curved lines indicate smoothed regression lines created using 95% of the data by smoothing function (LOWESS), short dashed lines are the quantile regressions of 10th and 90th percentiles of bias; and dashed horizontal lines represent a reference. Abbreviations: CKD-EPI, Chronic Kidney Disease Epidemiology Collaboration; CKD-EPI_PK, CKD-EPI equation with Pakistani correction factors; MDRD, Modification of Diet in Renal Disease.

Figure 2

Comparison of bias (measured glomerular filtration rate [GFR] − estimated GFR) for CKD-EPI_PK (CKD-EPI [Chronic Kidney Disease Epidemiology Collaboration] equation with Pakistani correction factors) equation and new estimating equations 1 and 2 among subgroups. Bars denote median bias, error bars represent standard error of median bias based on 1,000 bootstrapped samples. Bias values that are negative represent overestimation. Conversion factors for units: urine creatinine in g/d to mmol/d, ×8.84; urine urea nitrogen in g/d to mmol/d, ×35.7. Abbreviations and definitions: Neither, nonhypertensive nondiabetic; HTN only, hypertensive; DM only, diabetic; DM-HTN, diabetic and hypertensive.