Motivation: Tonight, at 8 pm, I was trudging up stairs with an empty stomach and one last task to do - looking at urine sediment under the microscope. But, is urine microscopy a useful heritage of the past? I was trying to use urine microscopy to distinguish pre-renal injury from acute tubular necrosis, which are often the most debated diagnoses in an inpatient setting. How good is urine microscopy for such a purpose? Turns out that these grumbling musings have been studied more conclusively in a prospective study.
Paper: Perazella, M.A., Cocoa, S.G., Kanbay, M. et. al. "Diagnostic Value of Urine Microscopy for Differential Diagnosis of Acute Kidney Injury in Hospitalized Patients." Clin J Am Soc Nephrol (2008) 3: 1615-1619.
Methods: At Yale-New Haven Hospital between April 2006 and May 2007, 267 inpatient nephrology consults were called for acute kidney injury. Prior to looking at urine sediment, the nephrologists were asked to provide a "pre-test" clinical diagnosis consisting of: (1) ATN, (2) pre-renal azotemia, or (3) other. Nephrologists could use all available information including trends in serum creatinine. Subsequently, after patient discharge, renal biopsy, or death, nephrologists were asked to provide a "final" diagnosis.
Results:
ATN vs Pre-renal: Overall in "final" diagnosis, 123 patients were diagnosed with ATN and 108 patients with prerenal AKI. Using the "final" diagnosis after hospital course as gold standard, the performance characteristics of urine microscopy in distinguishing ATN vs pre-renal AKI are as follows:
- sensitivity: 0.76
- specificity: 0.86
- positive likelihood ratio: 5.75
On further subgroup analysis, the authors looked at specific findings in urine microscopy most helpful in distinguishing ATN from pre-renal AKI (expressed in terms of likelihood ratio for ATN):
Granular Casts per High Power Field (40x magnification) +LR for ATN vs Prerenal:
0 ------------ LR: 0.23
1 to 5 ------- LR: 2.97
6 to 10 ------ LR: 9.68
>10 --------- LR: Infinite (No instances in pre-renal cases)
Note that granular casts did not have to be muddy brown!
Renal Tubular Cells per High Power Field +LR for ATN vs Prerenal:
0-------------- LR: 0.72
1 to 5 --------- LR: 1.97
6 to 20 -------- LR: Infinite (No instances in pre-renal cases)
>20 ----------- LR: Infinite (No instances in pre-renal cases)
Concordance: Between "pre-test" and "final" diagnosis, the concordance for diagnosis of pre-renal AKI was 77% while the concordance for ATN was 86%.
Discussion: I found this paper useful in a couple of different ways. First, I thought that the microscopic equivalent of ATN was muddy brown casts. Seeing a lot of granular casts or many renal tubular cells predicts ATN just as strongly! Secondly, urine microscopy remains useful. Overall initial clinical assessment of pre-renal AKI is wrong about a quarter of the time - an error rate that is pretty high. Observing many granular casts or renal tubular cells can certainly help reduce this error rate.
This paper, while helpful, has some curious methodological limitations. The authors had the nephrologists give a "pre-test" diagnosis. However, no "post-test" diagnosis was assessed after urine microscopy. That way, the effect of urine microscopy on diagnosis could be better assessed. Also, the "final" gold standard diagnosis used is an overall clinical assessment and not a renal biopsy, which could significantly change the diagnoses.
Paper: Perazella, M.A., Cocoa, S.G., Kanbay, M. et. al. "Diagnostic Value of Urine Microscopy for Differential Diagnosis of Acute Kidney Injury in Hospitalized Patients." Clin J Am Soc Nephrol (2008) 3: 1615-1619.
Methods: At Yale-New Haven Hospital between April 2006 and May 2007, 267 inpatient nephrology consults were called for acute kidney injury. Prior to looking at urine sediment, the nephrologists were asked to provide a "pre-test" clinical diagnosis consisting of: (1) ATN, (2) pre-renal azotemia, or (3) other. Nephrologists could use all available information including trends in serum creatinine. Subsequently, after patient discharge, renal biopsy, or death, nephrologists were asked to provide a "final" diagnosis.
Results:
ATN vs Pre-renal: Overall in "final" diagnosis, 123 patients were diagnosed with ATN and 108 patients with prerenal AKI. Using the "final" diagnosis after hospital course as gold standard, the performance characteristics of urine microscopy in distinguishing ATN vs pre-renal AKI are as follows:
- sensitivity: 0.76
- specificity: 0.86
- positive likelihood ratio: 5.75
On further subgroup analysis, the authors looked at specific findings in urine microscopy most helpful in distinguishing ATN from pre-renal AKI (expressed in terms of likelihood ratio for ATN):
Granular Casts per High Power Field (40x magnification) +LR for ATN vs Prerenal:
0 ------------ LR: 0.23
1 to 5 ------- LR: 2.97
6 to 10 ------ LR: 9.68
>10 --------- LR: Infinite (No instances in pre-renal cases)
Note that granular casts did not have to be muddy brown!
Renal Tubular Cells per High Power Field +LR for ATN vs Prerenal:
0-------------- LR: 0.72
1 to 5 --------- LR: 1.97
6 to 20 -------- LR: Infinite (No instances in pre-renal cases)
>20 ----------- LR: Infinite (No instances in pre-renal cases)
Concordance: Between "pre-test" and "final" diagnosis, the concordance for diagnosis of pre-renal AKI was 77% while the concordance for ATN was 86%.
Discussion: I found this paper useful in a couple of different ways. First, I thought that the microscopic equivalent of ATN was muddy brown casts. Seeing a lot of granular casts or many renal tubular cells predicts ATN just as strongly! Secondly, urine microscopy remains useful. Overall initial clinical assessment of pre-renal AKI is wrong about a quarter of the time - an error rate that is pretty high. Observing many granular casts or renal tubular cells can certainly help reduce this error rate.
This paper, while helpful, has some curious methodological limitations. The authors had the nephrologists give a "pre-test" diagnosis. However, no "post-test" diagnosis was assessed after urine microscopy. That way, the effect of urine microscopy on diagnosis could be better assessed. Also, the "final" gold standard diagnosis used is an overall clinical assessment and not a renal biopsy, which could significantly change the diagnoses.
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