You are following five patients who have had various aspects of their kidney function measured on one or more occasions over time. All patients are clinically stable without any ongoing acute events. Which of the following laboratory changes would represent the greatest loss in glomerular filtration rate?

Question

You are following five patients who have had various aspects of their kidney function measured on one or more occasions over time.  All patients are clinically stable without any ongoing acute events.  Which of the following laboratory changes would represent the greatest loss in glomerular filtration rate?

Quizplus · Accepted Answer

11ec193a_98f3_e1ab_853d_c98af6914a6d_MD0002_00 The relationship between serum creatinine concentration and glomerular filtration rate (GFR) is non-linear. As a rule of thumb, every time the GFR halves, the serum creatinine doubles. For example, if you assume that the GFR is 100 ml/min, the corresponding serum creatinine would be 1.0 mg/dL. As the GFR decreases to a value of 50 ml/min, the corresponding serum creatinine would be 2 mg/dL. With further decreases in GFR to 25, 12.5, and 6.25 ml/min, the corresponding serum creatinine concentrations would be 4.0, 8.0, and 16.0 mg/dL, respectively. This curvilinear relationship between GFR and serum creatinine is such that when the GFR is normal, relatively large decreases in GFR result in only small increases in serum creatinine (Choice A). Thus, when baseline serum creatinine is normal, serum creatinine is an insensitive indicator for decreasing GFR. (Choice B) When GFR is significantly decreased at baseline, small decrements in GFR produce relatively large changes in serum creatinine. The estimated loss of GFR with an increase in serum creatinine from 4.0 to 8.0 mg/dL is modest at about 12.5 ml/min. (Choices C & D) Blood urea nitrogen (BUN) is more affected than serum creatinine by factors other than GFR. For example, high protein diet, upper gastrointestinal hemorrhage, and the anti-anabolic effects of corticosteroids can increase BUN without affecting GFR and serum creatinine. Neither the BUN/creatinine ratio nor small changes in BUN are helpful indicators of GFR. (Choice E) A 24-hour urine collection for creatinine clearance is rarely done at present. Despite detailed instruction, it is difficult to obtain accurate collections. The 20 ml/min calculated decrease in creatinine clearance is modest and likely significantly less than that in choice A. Educational objective: The relationship between serum creatinine concentration and glomerular filtration rate (GFR) is non-linear; every time the GFR halves, the serum creatinine doubles. When the GFR is normal, relatively large decreases in GFR result in only small increases in serum creatinine. Conversely, when the GFR is significantly decreased, small decrements in GFR produce relatively large changes in serum creatinine. __________ References: How to measure renal function in clinical practice. (http://www.ncbi.nlm.nih.gov/pubmed/17023465)

You Are Following Five Patients Who Have Had Various Aspects