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Saudi Journal of Kidney Diseases and Transplantation
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Year : 2013  |  Volume : 24  |  Issue : 4  |  Page : 805-806
Narrowing the margins of errors intrinsic to the estimations made from uacr and upcr by introducing simple correction factors

King Fahad Specialist Hospital, P.O. Box 15215, Dammam - 31444, Kingdom of Saudi Arabia

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Date of Web Publication24-Jun-2013

How to cite this article:
Abutaleb N. Narrowing the margins of errors intrinsic to the estimations made from uacr and upcr by introducing simple correction factors. Saudi J Kidney Dis Transpl 2013;24:805-6

How to cite this URL:
Abutaleb N. Narrowing the margins of errors intrinsic to the estimations made from uacr and upcr by introducing simple correction factors. Saudi J Kidney Dis Transpl [serial online] 2013 [cited 2022 May 19];24:805-6. Available from: https://www.sjkdt.org/text.asp?2013/24/4/805/113899
To the Editor,

Given the wide variability in the 24-h protein excretion that can be associated with the same value of urinary protein or albumin/creatinine ratios (uACR/uPCR), [1] there is an obvious need for the introduction of a simple practical correction factor (CF) to ameliorate such uACR/ uPCR variability. The main reason for such proteinuria-independent variability in the values of uACR/uPCR is the urinary creatinine (UCr) variability, the denominator of the ratio, which reflects the variability in muscle mass among different individuals. Multiple physiological and pathological factors that affect muscle mass do affect UCr and, therefore, the uACR/uPCR values independent of the actual changes in albumin or protein excretion. In addition, as upright posture and physical activity may result in additional protein excretion, the uPCR may, however, fluctuate during the daytime. Morning UACR would help avoid the effect of many such factors, although the mid-morning uACR was reported to correlate better with daily UCr excretion. [1]

Calculated from the Cr index formula, a 70 kg young man would have a daily UCr excretion of 1700-1920 mg/d. [2] Whether derived from the suggested Cr index value of 23 (men) mg/ kg ideal body weight, [3] or from one of the age-adjusted Cr index equations, [4],[5] the average daily UCr excretion would come to significantly above 1 g/day presumed above. Even if we adopt the value quoted by Heymsfield et al [6] and earlier researchers of 1.36 g/day as the average daily urine Cr excretion for the normal 63.1 kg man, gross disparity in uACR is still expected. Such notions suggest significant general under-estimation of daily albuminuria/ proteinuria by both uACR and uPCR, respectively. In fact, the reader may have noted that National Kidney Foundation [1] has defined microalbuminuria (MAU) as 30 mg/day or as uACR of >17 (not >30!) and >25 in male and female patients, respectively. The choice of such two significantly different cut-off values reflects the significant gender effect in UCr, but it ignores the aging effect on muscle mass in addition to limiting the adjustment to the lower limit of MAU but ignoring the correction for uACR values across the whole range of albumin excretion.

Daily urine Cr excretion can be easily estimated for patients with different ages, weights and genders. Despite the shortcomings of the Crawford-Gualt (CG) equation, its predictive value for estimating Cr Cl is obviously better than its current use to assess renal function [not estimated glomerular filtration rate (eGFR)]. Botev et al [7] reported superior performance of CG over Modification of Diet in Renal Disease study in estimating GFR for patients in CKD stages 1 to 4. The failure of the CG equation in estimating GFR for patients in CKD stage 5 is mainly because of the high tubular creatinine excretion.

The suggested CF = UCr = (140 - age) × weight/5000 is derived from the CG equation for estimated Cr Cl. This equation to estimate the CF, which I independently derived from the CG equation, has been suggested by Dr. Ellam [8] and Dr. Ginsberg [9] to estimate UCr; though not offered as a CF for the urinary ratios values, Therefore, simply multiplying the uACR/uPCR values by the expected UCr (which is the CF value) will give the adjusted values that allow a more accurate estimation of daily urinary protein/albumin excretion. If the patient is female, the uACR/uPCR will also need to be multiplied by 0.85 as suggested by CG.

In my opinion, there is a need to improve the accuracy of interpreting uACR and uPCR. As the deviation of the UCr value from unity (1 g/day) would distort the ratio and its implication about the daily excretion, the correction factor, CF, that is being suggested would aim to correct the effect of the deviation of urinary creatinine from unity in metric units or from 10 mmol/day on using the International Units (IU).

I believe strongly that the CF formula, which retains the same structure as the CG equation, is practical and easy to memorize. In fact, it can be incorporated into the lab report to automatically produce adjusted ACR and PCR values. Clinical decisions, such as those to obtain a renal biopsy or refusing a potential kidney donor, can definitely be affected by such adjustments of ACR and PCR values. The adjustment would also be very helpful in refining the accuracy of CKD prevalence among the general population in CKD public studies. In patients with a previous measurement of daily UCr, CF would simply equal the value of UCr in g/day or 1/10 of the UCr value on using the IU.

The suggested CF would in fact correct the estimations of the daily excretion of all the substances measured in urine and corre-lated to urinary creatinine for the estimation of their daily excretion. Examples here would include calcium, potassium, oxalate, uric acid and many others. As these substances are correlated to simultaneous urinary creatinine concentration in the same way as albumin and protein in uACR/uPCR, the same suggested CF can be utilized according to the type of units utilized.

Conflict of interest: None declared.

   References Top

1.National Kidney Foundation, Managing Chronic kidney disease: A desk Reference, 2004.  Back to cited text no. 1
2.Oterdoom LH, van Ree RM, de Vries AP, et al. Urinary creatinine excretion reflecting muscle mass is a predictor of mortality and graft loss in renal transplant recipients. Transplantation 2008;86:391-8.  Back to cited text no. 2
3.Bistrian BR. Nutritional assessment and therapy of protein-calorie malnutrition in the hospital. J Am Diet Assoc 1977;71:393-7.  Back to cited text no. 3
4.Desmeules S, Lévesque R, Jaussent I, Leray-Moragues H, Chalabi L, Canaud B. Creatinine index and lean body mass are excellent predictors of long-term survival in haemodiafiltration patients. Nephrol Dial Transplant 2004;19:1182-9.  Back to cited text no. 4
5.Perrone RD, Madias NE, Levey AS. Serum creatinine as an index of renal function: New insights into old concepts. Clin Chem 1992; 38:1933-53.  Back to cited text no. 5
6.Heymsfield SB, Arteaga C, McManus C, Smith J, Moffitt S. Measurement of muscle mass in humans: Validity of the 24-hour urinary creatinine method. Am J Clin Nutr 1983;37:478-94.  Back to cited text no. 6
7.Botev R, Mallié JP, Couchoud C, et al. Estimating glomerular filtration rate: Cockcroft-Gault and modification of diet in renal disease formulas compared to renal inulin clearance. Clin J Am Soc Nephrol 2009;4:899-906.  Back to cited text no. 7
8.Ellam T. Improving the interpretation of protein: Creatinine ratios. The impact of creatinine excretion. Nephrol Dial Transplant 2011;26: 1108-9.  Back to cited text no. 8
9.Ginsberg JM, Chang BS, Matarese RA, Garella S. Use of single voided urine samples to estimate quantitative proteinuria. N Engl J Med 1983;309:1543-6.  Back to cited text no. 9

Correspondence Address:
Nasrulla Abutaleb
King Fahad Specialist Hospital, P.O. Box 15215, Dammam - 31444
Kingdom of Saudi Arabia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1319-2442.113899

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