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Score One for the Kidneys
Abstract & Commentary
By Allan J. Wilke, MD, Residency Program Director, Associate Professor of Family Medicine, University of Alabama at Birmingham School of Medicine—Huntsville Regional Medical Campus, Huntsville. Dr. Wilke reports no financial relationship to this field of study.
Synopsis: A simple questionnaire can identify individuals at risk for chronic kidney disease.
Source: Bang H, et al. SCreening for Occult REnal Disease (SCORED): a simple prediction model for chronic kidney disease. Arch Intern Med. 2007;167:374-381.
Hoping to increase the identification of patients with chronic kidney disease, Bang and colleagues used the National Health and Nutrition Examination Surveys (NHANES) dataset to develop a prediction tool. They combined data from the 1999-2000 and 2001-2002 surveys, identifying 10,291 subjects. Serum creatinine values and other covariates were missing on many, and after excluding them, the final dataset had 8,530 subjects. Glomerular filtration rate (GFR) was calculated with the abbreviated Modification of Diet in Renal Disease (MDRD) formula:
GFR (mL/min/1.73 m2) = 186 x Serum Creatinine (mg/dL)-1.154 x Age (years)-0.203 x 1.212 (if black) x 0.742 (if female).
They used GRF < 60 mL/min as the cutoff for chronic kidney disease (CKD). This is the value that the National Kidney Foundation uses as its definition of CKD and corresponds to Stage 3. Six hundred one of 8,530 (7%) had CKD. On multivariate analysis these variables were statistically significant: age, female gender, anemia, hypertension, diabetes, history of cardiovascular disease, history of congestive heart failure, peripheral vascular disease, and proteinuria. In developing the scoring system, the authors assigned 2 points for age 50-59, 3 points for age 60-69, and 4 points for age 70 or greater. All the other variables were assigned 1 point. They chose a score of 4 or greater as the cutoff for CKD because it had the best combination of sensitivity (92%) and specificity (68%). When they validated their rule on an independent dataset, the Atherosclerosis Risk in Communities (ARIC), a score of 4 or greater identified 36% of people with kidney disease. The positive predictive value (PPV) was 18% and the negative predictive value (NPV) was 99%.
It is estimated that there are 20 million people in the US with CKD.1 In 2002, the National Kidney Foundation published clinical guidelines for the prevention, diagnosis, and treatment of CKD.2 Authors of a study3 done in Norway found a similar percentage of individuals with CKD (same definition as above) when they screened people with hypertension, diabetes mellitus, or age > 55. Of the people who screened positive, only 1% progressed to end-stage renal disease (CKD Stage 5).
You may be wondering where this rule would be used, since most laboratories will calculate and report back GFR on patients who have had a serum creatinine drawn. The authors envisioned the tool being used "to identify individuals with a high likelihood of CKD before any evaluation with a serum laboratory analysis." They suggest that this could occur at mass screenings or it could be used in primary care offices by intake staff. They also suggest it could be used by patients on interactive websites. The authors include a sample questionnaire for risk evaluation. If a patient scored 4 or more, then they would have a confirmatory serum creatinine drawn.
The equation for the abbreviated MDRD formula is not one that I can keep in my head, but that's why personal digital assistants were invented. The National Kidney Foundation (www.kidney.org/professionals) and Epocrates (www.epocrates.com/index.html) are sources for downloadable PDA programs that will do the math for you. Those of you who use one of these or who looked closely at the equation will note that race (specifically being African American) is a variable, but was not noted to be a risk factor in this study. The authors were not able to explain this. Using this tool with a cutoff of 4 puts everyone 70 years or older in the at-risk group. The implication is that there's no reason to use it in that age group, just get a serum creatinine on everyone.
It's been two decades since Frame set forth his criteria for a good screening test.4 They are:
1. The condition must have a significant effect on the quality or quantity of life.
2. Acceptable methods of treatment must be available.
3. The condition must have an asymptomatic period during which detection and treatment significantly reduce morbidity or mortality.
4. Treatment in the asymptomatic phase must yield a therapeutic result superior to that obtained by delaying treatment until symptoms appear.
5. Tests that are acceptable to patients must be available at reasonable cost to detect the condition in the asymptomatic period.
6. The incidence of the condition must be sufficient to justify the cost of screening.
CKD is a condition and SCORED is a screening test that meets these criteria. I will be introducing it to our office.
1. Coresh J, et al. J Am Soc Nephrol. 2005;16:180-188.
2. National Kidney Foundation. K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Am J Kidney Dis. 2002;39(2 Suppl 1):S1-266.
3. Hallan SI, et al. BMJ. 2006;333:1047.
4. Frame PS. J Fam Pract. 1986;22:341-346. Review.