Sultan Qaboos University Med J, February 2014, Vol. 14, Iss. 1, pp. e72-79, Epub. 27TH Jan 14 Submitted 8TH May 13 Revision Req. 21ST Jun 13; Revision recd. 30TH Jul 13 Accepted 25th Aug 13 1Department of Internal Medicine, Al Nahdha Hospital, Muscat; 2Ministry of Health, Muscat, Oman *Corresponding Author e-mail: mag_alosali@yahoo.com تقييم سرعة الرتشيح الكبييب ىف املرضى العمانيني باستخدام معادلة كوككروفت-جولت و معادلة تعديل النظام الغذائى ملرضى الكلى جمدي عيد �لع�صيلي و �صامل �صعيد �لق�صابى و �صعود حممد �حلارثي abstract: Objectives: Glomerular filtration rate (GFR) is the best index of renal function and is frequently assessed by corrected creatinine clearance (CCLcr). The limitations of CCLcr have inspired researchers to derive easy formulas to estimate GFR, with Cockcroft-Gault (C-G) and the modification of diet in renal disease (MDRD) being the most widely used. This study aimed to evaluate the validity of these equations by finding the relation between CCLcr and estimated GFR (eGFR) by C-G, modified C-G and MDRD equations. Methods: From 2007 to 2011, 158 subjects were analysed for serum creatinine and CCLcr at Bowsher Polyclinic, Muscat, Oman. The C-G equation was used to obtain eGFRC-G which was adjusted to body surface area (BSA) to obtain eGFRmC-G, and the MDRD equation was used to obtain eGFRMDRD. The eGFRMDRD, eGFRmC-G and eGFRC-G were then compared to CCLcr. Results: The eGFRMDRD, eGFRmC-G and eGFRC-G significantly correlated with CCLcr, with a slightly stronger correlation with eGFRMDRD (r = 0.701, 0.658 and 0.605, respectively). A receiver operating characteristic curve analysis showed that the diagnostic accuracy of eGFRMDRD for diagnosing chronic kidney disease (CKD) was higher than that of eGFRmC-G, which in turn was higher than that of eGFRC-G (area under the curve was 0.846, 0.831, and 0.791; cut-off limits were 61.9, 58.3 and 59.5, respectively). Conclusion: C-G and MDRD equations can be an alternative to the CCLcr test for assessing GFR, thus avoiding the need for the cumbersome and expensive GFR test. The MDRD formula had greater validity than the C-G equation and the C-G equation validity was improved by an adjustment to BSA. Keywords: Creatinine; Glomerular Filtration Rate; Diet Modification; Chronic Kidney Disease; Oman. امللخ�ص: الهدف: يعد ح�صاب �رشعة �لرت�صيح �لكبيبي و�لذي يقا�س عن طريق ��صتخال�س �لكرياتيني من �أف�صل �ملوؤ�رش�ت لوظائف �لكليتي. ونظر� ل�صلبيات معادلة تعد و �لكرياتيني. ��صتخال�س على تعتمد ال �لكبيبي �لرت�صيح �رشعة حل�صاب معادالت ال�صتقاق ��صطر�لباحثون �لكرياتيني, ��صتخال�س طريقة عن �ملعادالت هذه تقييم هو �لدر��صة هذه من �لغر�س �صيوعا. �ملعادالت هذه �أكرث من �لكلى ملر�صى �لغذ�ئي �لنظام تعديل ومعادلة كوككروفت-جولت طريق �إيجاد �لعالقة بي ��صتخال�س �لكرياتيني وكل من �رشعة �لرت�صيح �لكبيبي عن طريق معادلة كوككروفت-جولت و �رشعة �لرت�صيح �لكبيبي عن طريق لقد الطريقة: �لكلى. ملر�صى �لغذ�ئي �لنظام تعديل معادلة طريق عن �لكبيبي �لرت�صيح �رشعة و �جل�صم �صطح مل�صاحة �ملعدلة كوككروفت-جولت معادلة �أجريت هذه �لدر��صة يف جممع بو�رش �لتخ�ص�صي يف حمافظة م�صقط يف عمان )�لفرتة من عام 2007و حتى عام 2011(. و مت قيا�س ن�صبة �لكرياتيني بالدم و كمية ��صتخال�س �لكرياتيني يف بول 24 �صاعة يف �ملر�صى �مل�صاركي يف �لدر��صة و عددهم مائة و ثمانية و خم�صون مري�صا. مت ح�صاب �رشعة �لرت�صيح �لكبيبي عن طريق معادلة كوككروفت-جولت و�رشعة �لرت�صيح �لكبيبي عن طريق معادلة كوككروفت-جولت �ملعدلة مل�صاحة �صطح �جل�صم و �رشعة �لرت�صيح �لكبيبي عن طريق معادلة تعديل �لنظام �لغذ�ئي ملر�صى �لكلى جلميع �ملر�صى ثم مت در��صة �لعالقة بي ��صتخال�س �لكرياتيني و بي و �رشعة �لرت�صيح �لكبيبي عن طريق �ملعادالت �لثالثة. النتائج: تبي �نه يوجد �رتباط كبريبي كل من �رشعة �لرت�صيح �لكبيبي عن طريق معادلة تعديل �لنظام �لغذ�ئي ملر�صى �لكلى و�رشعة �لرت�صيح �لكبيبي عن طريق معادلة كوككروفت-جولت �ملعدلة و�رشعة �لرت�صيح �لكبيبي عن طريق معادلة كوككروفت-جولت و بي ��صتخال�س �لكرياتيني)معامل �الرتباط= 0.701 و0.658 و0.605 بالرتتيب( و �أظهر منحنى خ�صائ�س �لت�صغيل �أن قدرة �رشعة �لرت�صيح �لكبيبي عن طريق معادلة تعديل �لنظام �لغذ�ئى ملر�صى �لكلى لت�صخي�س مر�س �لكلى �ملزمن �أعلى من قدرة �رشعة �لرت�صيح �لكبيّب عن طريق معادلة كوككروفت-جولت �ملعدلة مل�صاحة �صطح �جل�صم و �لذي بدوره �أعلى من �رشعة �لرت�صيح �لكبيبي عن طريق معادلة كوككروفت- جولت )�ملنطقه حتت �ملنحنى كانت 0.846و 0.831و0.791و حدود �لقطع كانت 61.9 و58.3 و59.5 بالرتتيب( اخلال�سة: تعد معادلة كوككروفت-جولت و معادلة تعديل �لنظام �لغذ�ئى ملر�صى �لكلى بديل عن �ختبار ��صتخال�س �لكرياتيني لقيا�س �رشعة �لرت�صيح �لكبيبي مما يوؤدي �إىل جتنب �صعوبة و تكلفة هذ� �الختبار. وتعد معادلة تعديل �لنظام �لغذ�ئى ملر�صى �لكلى �أكرث دقة من معادلة كوككروفت-جولت �ملعدلة مل�صاحة �صطح �جل�صم و �لتي بدورها �أكرث دقة من معادلة كوككروفت-جولت �الأ�صلية. مفتاح الكلمات: �لكرياتيني؛ �رشعة �لرت�صيح �لكبيبي؛ تعديل �لنظام �لغذ�ئي؛ مر�س �لكليتي �ملزمن؛ عمان. Assessment of Glomerular Filtration Rates by Cockcroft-Gault and Modification of Diet in Renal Disease Equations in a Cohort of Omani Patients *Magdi E. Al-Osali,1 Salim S. Al-Qassabi,2 Saud M. Al-Harthi2 CLINICAL & BASIC RESEARCH Magdi E. Al-Osali, Salim S. Al-Qassabi and Saud M. Al-Harthi Clinical and Basic Research | e73 Glomerular filtration rate (GFR) is considered the best index of renal function as it assesses the progression of kidney dysfunction. The normal value is ~130 and 120 ml/min/1.73 m² for men and women, respectively, depending on age, sex and body size.1 GFR can be determined by measuring the clearance of exogenous (inulin, 125-iothalamate, 51 Cr- ethylene diamine tetra acetic acid [EDTA], 99mTc- diethylene triamine penta acetic acid [DTPA] and iohexol) or endogenous (creatinine) substances.2 Methods using exogenous substances are expensive, time-consuming, risky and cannot be easily implemented in clinical practice. Nevertheless, inulin clearance is the gold standard test for GFR as it is freely filtered and is not secreted, reabsorbed, synthesised or metabolised by the kidney.3 Creatine clearance (CLcr) is an alternative to inulin clearance. Creatinine is freely filtered and is not metabolised by the kidney; however, it is secreted by the renal tubules.4 If the effect of secretion is ignored, then all of the filtered creatinine will be excreted and this will reflect the GFR. Thus the GFR and CLcr will be equal: [UCr x V]/SCr,5 where UCr is urine creatinine, V is the 24- hour urine volume and SCr is the serum creatinine. However, CLcr tends to exceed the true GFR due to tubular secretion.5 It should therefore be adjusted to body surface area (BSA) so as to obtain the corrected creatinine clearance (CCLcr) in ml/min/1.73 m² by the following equation:6 The normal value of CCLcr is 95 ± 20 ml/min per 1.73 m² in women and 120 ± 25 ml/min per 1.73 m2 in men.5 SCr varies inversely with GFR and is used to assess stable kidney function, as a rise in SCr represents a reduction in GFR. However, in acute renal failure, GFR is markedly reduced and there is no time for creatinine to accumulate.6 The mean SCr values for men and women are 100 and 82 µmol/L, respectively. These values vary by race and differ according to its production, secretion, extrarenal excretion and assay.7,8 The limitations of CLcr and inulin clearance have inspired researchers to seek out easy formulas to estimate GFR (eGFR).9 The most widely used formulas are Cockcroft-Gault (C-G)10 and the modification of diet in renal disease (MDRD).11 These formulas include variables such as age, sex, race, weight and SCr. In adults, normal eGFR is ≥90 ml/min/1.73m². Chronic kidney disease (CKD) is defined by an eGFR of <60 ml/min/1.73 m2.9 As for SCr, the proper interpretation of these equations requires stable kidney function, and its accuracy is also limited as SCr is affected by factors other than creatinine filtration.12,13 In the C-G equation, CLcr can be estimated by the following formula:10 This formula should be adjusted for BSA to increase its accuracy and compare normal values.14 It appears to be less accurate in the obese, those of different ethnicities, different age groups, children and pregnant women.1 The original MDRD equation has six variables, including urea and albumin which was a limitation Advances in Knowledge - Methods using exogenous substances to assess renal function are expensive, time-consuming, risky and cannot be easily implemented in clinical practice. Additionally, creatinine clearance (CLcr) has some limitations. Due to the limitations of the clearance tests, they are frequently replaced by estimation equations such as Cockcroft-Gault (C-G) and the modification of diet in renal disease (MDRD) formulas. - In this study, the MDRD formula had greater validity than the C-G equation and the C-G equation validity was improved by an adjustment to body surface area. Applications to Patient Care - The knowledge that C-G and MDRD equations can be used as an alternative to the CLcr test for assessing GFR will enable the patient to avoid the time-consuming, cumbersome and expensive CLcr test. - It will be easier for the clinician to follow the progress of kidney disease by assessing eGFR with these equations, depending on patient age, weight and serum creatinine. It will also circumvent the need for 24-hour urine collection. - The MDRD formula is recommended to assess eGFR in patients with chronic kidney disease as it has greater validity than the C-G equation. CCLcr = (CLcr x 1.73) BSA CLcr (ml/min) = (140 - age in years ) x (weight in Kg) x 1.23 if male (1.04 if female)/SCr in µmol/L Assessment of Glomerular Filtration Rates by Cockcroft-Gault and Modification of Diet in Renal Disease Equations in a Cohort of Omani Patients e74 | SQU Medical Journal, February 2014, Volume 14, Issue 1 eGFR (ml/min per 1.73 m2) = 1.75 x SCr-1.154 x age- 0.203 x 1.212 (if of African descent) x 0.742 (if female), where SCr is in µmol/L and age in years for the added cost and analytical variation.13 Recognising this, the MDRD-4 variable equation was developed based on SCr, age, gender and ethnicity by the following formula:15 This study was conducted primarily to evaluate the performance of C-G and MDRD equations in Omani patients by finding out the relation between CCLcr and eGFR by using C-G (eGFRC-G), modified C-G (eGFRmC-G ) and MDRD (eGFRMDRD) equations. Secondly, we sought to replace the CLcr test with eGFR for the assessment of kidney function in clinical practice, thereby avoiding the need for the time-consuming, cumbersome and expensive CLcr test. Methods This cross-sectional analytical study was carried out at Bowsher Polyclinic, Muscat, Oman, by auditing the files of subjects reporting to the Internal Medicine Clinic for a CLcr test to assess kidney function from 1 January 2007 to 30 April 2011. Ethical approval was received from the Regional Research & Ethics Committee of the Directorate General & Health Services of the Muscat Region. The inclusion criteria included adult patients who reported to the Internal Medicine Clinic at Polyclinic for a CLcr test. However, patients who had incomplete data or dialysis therapy were excluded; thus 97 of the 255 files reviewed could not be considered, leaving a total of 158 subjects. Demographic data, such as age, gender, weight, height, body mass index (BMI) and BSA, were recorded. All subjects were analysed for SCr and subjected to 24-hour urine collection to estimate urine volume (V) and urine creatinine (UCr). The CLcr was calculated by the following equation:5 The CLcr was then adjusted to BSA to get CCLcr in ml/min per 1.73 m² by the following formula, where BSA equals the square root ([height in cm x weight in Kg]/3600):8,16 Depending on a patient’s gender, age and SCr, C-G was used to obtain the predicted CLcr, which was abbreviated as eGFRC-G, as in the following formula:10 The eGFRC-G (ml/min) was adjusted to BSA (modified C-G) to obtain eGFRmC-G (ml/min per 1.73 m²): eGFRmC-G = eGFRC-G x 1.73/BSA. The MDRD-4 variable equation was used to obtain eGFRMDRD in ml/min per 1.73 m² by the following formula:15 The eGFRC-G, eGFRmC-G and eGFRMDRD were compared to CCLcr and statistical analysis was done to find out the correlation between them and to estimate an agreement between them. Data were coded using the Statistical Package for the Social Sciences (SPSS), Version 15 (IBM, Corp., Chicago, Illinois, USA) and summarised using the mean, standard deviation (SD), minimal and maximum values for quantitative variables and number and percentage for qualitative values. Correlations were done to test for linear relations between variables. Logistic regression analysis was done to test for significant predictors of dependent variables. A receiver operating characteristic (ROC) curve was used to test the validity of scores calculated by regression equations. A P value of ≤0.05 was considered statistically significant. Results The subjects in the study (N = 158) were predominantly <70 years of age (n = 115), although 43 were ≥70 years. The gender distribution was nearly equal (85 males and 73 females) and 42 were obese while 116 were not considered obese. Of those included in the study, 99 had diabetes (DM) and 59 were non-diabetic. The mean ± SD (range) age was 61.65 ± 10.46 (34.0–82.0); BMI was 27.93 ± 5.89 (16.6–54.6); SCr was 108.23 ± 47.12 (28.0–373.0); CCLcr was 69.52 ± 37.28 (10.30–196.5); eGFRMDRD was 62.89 ± 27.52 (14.0–206.0); eGFRmC-G was 66.37 ± 28.09 (18.3–154.3), and eGFRC-G was 66.87 ± CCLcr =(CLcr x 1.73) BSA CCLcr (ml/min)=(UCr x V) SCr eGFRC-G (ml/min) = (140 - age in years) x (weight in Kg) x 1.23 if male (1.04 if female )/SCr in µmol/L. eGFRMDRD = 175 x SCr-1.154 x age-0.203) x 1.212 (if of African descent) x 0.742 (if female), where SCr is in µmol/L and age in years. None of our patients were of African descent. Magdi E. Al-Osali, Salim S. Al-Qassabi and Saud M. Al-Harthi Clinical and Basic Research | e75 30.54 (20.21–163.92) of the studied subjects. The eGFRMDRD, eGFRmC-G and eGFRC-G correlated significantly with CCLcr, with a slightly stronger correlation with eGFRMDRD (r = 0.701, 0.658 and 0.605, respectively; P <0.001). Studying eGFRMDRD, eGFRmC-G and eGFRC-G at a known cut-off value of 90 found that eGFRmC-G had a higher validity than eGFRC-G and that eGFRMDRD had a higher sensitivity and lower specificity than either eGFRmC-G or eGFRC-G (sensitivity = 97.4, 93.6 and 92.3; specificity = 22.5%, 27.5% and 26.3%, respectively). The ROC curve analysis showed that the diagnostic accuracy of eGFRmC-G for a diagnosis of CKD was higher than that of eGFRC-G. The eGFRMDRD had a higher area under the curve (AUC) and higher sensitivity and lower specificity than either eGFRC-G or eGFRmC-G [Figure 1 and Table 1]. Regression analysis was performed to predict renal impairment by using eGFRC-G adjusted for age, sex, obesity and DM. A regression equation was applied to calculate the predicted score for each patient (ranging from 0–100). The predicted score was entered in a ROC curve to detect its validity as well as to determine the best cut-off value for diagnosing renal impairment. The same was done for eGFRmC-G and eGFRMDRD for comparison. A ROC curve analysis showed that the eGFRmC-G score had a higher AUC, sensitivity, negative predictive value (NPV) and total accuracy (TA), and lower specificity and positive predictive value (PPV) than the eGFRC-G score. Additionally, the eGFRMDRD score had a higher validity than the eGFRmC-G score [Figure 2 and Table 2]. Regarding the validity among the studied groups, the eGFRMDRD had a higher validity than either eGFRC-G or eGFRmC-G in the obese, diabetic, male or the ≥70-year-old subjects. Comparing the validity of eGFRmC-G and eGFRC-G, this study also showed that eGFRmC-G had higher validity in the Table 1: The validity of eGFRC-G, eGFRmC-G and eGFRMDRD as a diagnostic tool for renal impairment after receiver operating characteristic curve analysis AUC P value Cut-off values* Sensitivity Specificity PPV NPV TA eGFRC-G 0.791 <0.001 ≤59.5 73.1 80.0 78.1 75.3 76.6 eGFRmC-G 0.831 <0.001 ≤58.3 75.6 85.0 83.1 78.2 80.4 eGFRMDRD 0.846 <0.001 ≤61.9 82.1 72.5 74.4 80.6 77.2 AUC = area under the curve; PPV = positive predictive value; NPV = negative predictive value; TA = total accuracy; eGFRC-G = estimated glomerular filtration rate by Cockcroft-Gault equation; eGFRmC-G = estimated glomerular filtration rate by modified Cockcroft-Gault equation; eGFRMDRD = estimated glomerular filtration rate by modification of diet in renal disease. *mg/min for eGFRC-G and mg/min/1.73 m2 for eGFRmC-G and eGFRMDRD . Figure 1: The validity of eGFRC-G , eGFRmC-G and eGFRMDRD as a diagnostic tool for renal impairment after receiver operating characteristic curve analysis. eGFRC-G = estimated glomerular filtration rate by Cockcroft- Gault equation; eGFRmC-G = estimated glomerular filtration rate by modified Cockcroft-Gault equation; eGFRMDRD = estimated glomerular filtration rate by modification of diet in renal disease. Figure 2: Receiver operating characteristic curve for eGFRC-G , eGFRmC-G and eGFRMDRD for the assessment of kidney function after adjustment for age, sex, weight and diabetes mellitus*. eGFRC-G = estimated glomerular filtration rate by Cockcroft- Gault equation; eGFRmC-G = estimated glomerular filtration rate by modified Cockcroft-Gault equation; eGFRMDRD = estimated glomerular filtration rate by modification of diet in renal disease. *Predicted probability 1 by eGFRC-G ; predicted probability 2 by eGFRmC-G ; predicted probability 3 by eGFRMDRD . Assessment of Glomerular Filtration Rates by Cockcroft-Gault and Modification of Diet in Renal Disease Equations in a Cohort of Omani Patients e76 | SQU Medical Journal, February 2014, Volume 14, Issue 1 ≥70-year-old, male and diabetic subjects; however, in the obese subjects, eGFRmC-G was more sensitive but had less specificity, PPV, NPV and TA than in eGFRC-G [Table 3]. Discussion GFR is the best index of renal function in health and disease. It can be estimated by measuring the renal clearance of certain substances using exogenous (radioisotopic and non-radioisotopic) filtration markers. However, these methods are impractical and expensive.17 Endogenous markers such as creatinine have also been used to assess GFR. The accuracy of CLcr may be limited by inaccurate urine collection and creatinine secretion. Not only is urine collection time-consuming and cumbersome, but incomplete collection leads to a reduced CLcr while over-collection leads to an increased CLcr.8 Moreover, CLcr overestimates the GFR due to tubular creatinine secretion.5 To compensate for these previous limitations, investigators have devised equations that predict GFR based on SCr, gender, body size, race and age. The most widely used equations are the C-G equation, which produces GFR values in ml/min, and the MDRD equation, which produces GFR values in ml/min per 1.73 m².18 The C-G equation should be adjusted for BSA to increase its accuracy and enable a comparison with normal values.14 In this study, we evaluated the performance of the C-G and MDRD equations for estimating the GFR in a cohort of 158 subjects. An important characteristic of the cohort is that it included subjects whose CCLcr ranged from 10.3–196.5 ml/ min per 1.73 m² with sufficient numbers of subjects having CCLcr >60 and <60 (84 and 74, respectively). Thus, the performance of these equations could be assessed over a wide range of kidney function. Furthermore, because all patients included in this study were Arab, the performances of the C-G and MDRD equations could be assessed in a group of subjects whose anthropometric characteristics are slightly different from those of American or European subjects. With these different anthropometric characteristics in mind, we compared eGFRMDRD, eGFRmC-G and eGFRC-G with CCLcr. It was found that these equations underestimated GFR in comparison to CCLcr (mean CCLcr, eGFRMDRD, eGFRmC-G and eGFRC-G were 69.52, 62.89, 66.37 and 66.87, respectively). This can be explained by the fact that CCLcr exceeds the true GFR by 19% because of tubular secretion.5 In their study, Froissart et al. showed that there was a very good global agreement between measured GFR and both eGFRMDRD and eGFRmC-G. On average, eGFRMDRD was only 1.0 ml/ min per 1.73 m² less than measured GFR; eGFRmC-G was only 1.9 ml/min per 1.73 m² greater than measured GFR. However, Froissart et al.’s study compared eGFRMDRD and eGFRmC-G against GFR measured by 51Cr-EDTA renal clearance, and not CCLcr, and did not evaluate eGFRC-G.19 Similarly, in 1999, Levey et al. documented that the C-G formula largely overestimated measured GFR.13 The current study demonstrated that eGFRMDRD, eGFRmC-G and eGFRC-G can replace CCLcr in practice, avoiding the limitations of CCLcr, as evidenced by the significant correlation between them, with a stronger correlation with eGFRMDRD (r = 0.701, 0.658 and 0.605, respectively; P <0.001). These results are supported by a Pakistani study which compared eGFRMDRD and eGFRC-G with CCLcr in 369 cases, revealing a significant correlation between them, with a stronger correlation with eGFRMDRD (r = 0.788 for eGFRMDRD and r = 0.775 for eGFRC-G). However, that study did not evaluate eGFRmC-G.18 In 2006, Shoker et al. compared Table 2: The validity of eGFRC-G, eGFRmC-G and eGFRMDRD as a diagnostic tool for the assessment of kidney function after adjustment for age, sex, weight and diabetes AUC P value Cut-off values* Sensitivity Specificity PPV NPV TA eGFRC-G 0.806 <0.001 ≥48.7 80.8 73.8 75.0 79.7 77.2 eGFRmC-G 0.841 <0.001 ≥46.3 84.6 71.3 74.2 82.6 77.8 eGFRMDRD 0.853 <0.001 ≥48.4 84.6 73.8 75.9 83.1 79.1 AUC = area under the curve; PPV = positive predictive value; NPV = negative predictive value; TA = total accuracy; eGFRC-G = estimated glomerular filtration rate by Cockcroft-Gault equation; eGFRmC-G = estimated glomerular filtration rate by modified Cockcroft-Gault equation; eGFRMDRD = estimated glomerular filtration rate by modification of diet in renal disease. *mg/min for eGFRC-G and mg/min/1.73 m2 for eGFRmC-G and eGFRMDRD . Magdi E. Al-Osali, Salim S. Al-Qassabi and Saud M. Al-Harthi Clinical and Basic Research | e77 eGFRmC-G and eGFRC-G with CCLcr, documenting that eGFRmC-G gave superior results compared to eGFRC-G, with an overall accuracy in the general and subgroup analysis.14 Similarly, our results showed that eGFRmC-G had a stronger correlation with CCLcr than eGFRC-G emphasising that the correction for BSA increases the validity of the C-G equation. The difference between the two studies is that eGFRmC-G and eGFRC-G were compared with CLcr in the Shoker et al. study, but in our study they were compared with CCLcr, which is more accurate. In 2012, Alcântara et al. compared eGFRC-G with CCLcr and no significant difference was found between the mean eGFRC-G (64.7 ± 27.4) and the mean CCLcr (68.4 ± 32.6) and a correlation between them was found (r = 0.68; P <0.001). Using lean body weight instead of total body weight to obtain the eGFRC-G, the correlation coefficient was increased to 0.75 (P <0.001).20 However, Alcântara et al.’s study did not evaluate eGFRmC-G and eGFRMDRD, as in our study. In studying eGFRMDRD, eGFRmC-G and eGFRC-G as a diagnostic tool for renal impairment, as Table 3: The validity of eGFRC-G, eGFRmC-G and eGFRMDRD in diagnosing renal impairment among different studied groups Variable eGFR Group Sensitivity Specificity PPV NPV TA Age eGFRC-G <70 76.5 84.4 79.6 81.8 80.9 eGFRmC-G <70 82.4 81.3 77.8 85.2 81.7 eGFRMDRD <70 82.4 79.7 76.4 85.0 80.9 eGFRC-G ≥70 88.9 31.3 68.6 62.5 67.4 eGFRmC-G ≥70 88.9 31.3 68.6 62.5 67.4 eGFRMDRD ≥70 88.9 50.0 75.0 72.7 74.4 Sex eGFRC-G F 83.8 72.2 75.6 81.3 78.1 eGFRmC-G F 83.8 72.2 75.6 81.3 78.1 eGFRMDRD F 83.8 72.2 75.6 81.3 78.1 eGFRC-G M 78.0 75.0 74.4 78.6 76.5 eGFRmC-G M 85.4 70.5 72.9 83.8 77.6 eGFRMDRD M 85.4 75.0 76.1 84.6 80.0 BMI eGFRC-G Not 81.8 78.7 77.6 82.8 80.2 eGFRmC-G Not 85.5 77.0 77.0 85.5 81.0 eGFRMDRD Not 83.6 77.0 76.7 83.9 80.2 eGFRC-G Obese 78.3 57.9 69.2 68.8 69.0 eGFRmC-G Obese 82.6 52.6 67.9 71.4 69.0 eGFRMDRD Obese 87.0 63.2 74.1 80.0 76.2 DM eGFRC-G DM 87.3 56.8 71.6 78.1 73.7 eGFRmC-G DM 89.1 56.8 72.1 80.6 74.7 eGFRMDRD DM 89.1 61.4 74.2 81.8 76.8 eGFRC-G No DM 65.2 94.4 88.2 81.0 83.1 eGFRmC-G No DM 73.9 88.9 81.0 84.2 83.1 eGFRMDRD No DM 73.9 88.9 81.0 84.2 83.1 eGFR = estimated glomerular filtration rate; PPV = positive predictive values; NPV = negative predictive value; TA = total accuracy; eGFRC-G = estimated glomerular filtration rate by Cockcroft-Gault equation; eGFRmC-G = estimated glomerular filtration rate by modified Cockcroft-Gault equation; eGFRMDRD = estimated glomerular filtration rate by modification of diet in renal disease; BMI = body mass index; F = female; M = male; DM = diabetes mellitus. Assessment of Glomerular Filtration Rates by Cockcroft-Gault and Modification of Diet in Renal Disease Equations in a Cohort of Omani Patients e78 | SQU Medical Journal, February 2014, Volume 14, Issue 1 detected by CCLcr and at a known cut-off value of 90, it was found that eGFRmC-G had a higher validity than eGFRC-G. This emphasises that correction for BSA increases the validity of the C-G equation and that eGFRMDRD had a higher sensitivity and lower specificity than either eGFRmC-G or eGFRC-G. A ROC curve analysis showed that the diagnostic accuracy of eGFRmC-G for diagnosing CKD was higher than that of eGFRC-G, and that eGFRMDRD had a higher sensitivity, higher AUC and a lower specificity than either eGFRC-G or eGFRmC-G. By doing a regression analysis to predict renal impairment, using eGFRC-G, eGFRmC-G and eGFRMDRD adjusted for age, sex, obesity and DM, the ROC curve analysis showed that the eGFRmC-G score had a higher AUC, sensitivity, NPV and TA, and a lower specificity and PPV than that of the eGFRC-G score. Additionally, it showed that the eGFRMDRD score had a higher validity than the eGFRmC-G score. Our results supported those of Srinivas et al., whose study compared eGFRMDRD and eGFRmC-G with GFR measured by 99mTc-DTPA renal clearance in 599 renal donors; this study demonstrated that eGFRMDRD performed better in terms of global bias, precision, correlation and accuracy than eGFRmC-G.21 Regarding the validity among studied groups, our study showed that eGFRMDRD had a higher validity than either eGFRC-G or eGFRmC-G in males, those with DM, individuals ≥70 years of age and those who were obese. The eGFRmC-G had higher validity in diabetics, males and those ≥70 years of age than eGFRC-G; however, in the obese subjects, eGFRmC-G was more sensitive but had less specificity, PPV, NPV and TA than eGFRC-G. This was similar to Froissart et al.’s study, which showed that eGFRmC-G had the lowest level of precision for obese subjects.19 In 2005, Rigalleau et al. compared eGFRMDRD and eGFRmC-G with measured GFR in 160 diabetic patients, and revealed that eGFRMDRD and eGFRmC-G correlated well with measured GFR, while eGFRMDRD underestimated and eGFRmC-G overestimated it. The ROC curve analysis showed that the maximum diagnostic accuracy of eGFRmC-G for diagnosing CKD was lower than that of eGFRMDRD. It was concluded that the MDRD equation is more accurate for the diagnosis of renal failure in diabetic patients.22 However, eGFRMDRD and eGFRmC-G were evaluated against measured GFR by 51Cr-EDTA clearance and not against CCLcr. The eGFRC-G was not evaluated. Based on the current study, as well as other studies, it is clear that the measurement of CLcr using a 24-hour urine collection system does not improve the estimate of GFR compared to that provided by the C-G and MDRD equations. Nevertheless, this system provides useful information for the estimation of GFR in individuals with unsual dietary intake (for example in subjects with vegetarian diets or those taking creatine supplements), or abnormal muscle mass (for instance as a result of amputation, malnutrition or muscle wasting). It is also useful for the assessment of diet and nutritional status, and for assessing the patient’s status when there is a need to start dialysis.9 There are several limitations to this study. First, CLcr was used as the reference method for GFR although the measurement of CLcr has many theoretical and practical difficulties. Ideally it should be substituted by inulin or isotope clearances as a reference to verify the accuracy of the results. 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