Exercise: Test for urinary tract infection

This website is for students following the M.Sc. programme in Health Sciences at the University of York.

The following is the abstract of a paper:

The aim of this study was to evaluate the diagnostic properties of urine Gram stain and urine microscopic examination for screening for urinary tract infection (UTI), and to perform an additional cost utility analysis. This descriptive study was performed on 95 urine samples sent for urine culture to the Department of Microbiology, Faculty of Medicine, Chulalongkorn University. The first part of the study was to determine the diagnostic properties of two screening tests (urine Gram stain and urine microscopic examination). Urine culture was set as the gold standard and the results from both methods were compared to this. The second part of this study was to perform a cost utility analysis.

The sensitivity of urine Gram stain was 96.2%, the specificity 93.0%, the positive predictive value 94.3% and the negative predictive value 95.2%. False positives occurred with a frequency of 7.0% and false negatives 3.8%. For the microscopic examination, the sensitivity was 65.4%, specificity 74.4%, positive predictive value 75.6% and negative predictive value 64.0%. False positives occurred with a frequency of 25.6% and false negatives 34.6%. Combining urine Gram stain and urine microscopic examination, the sensitivity was 98.1%, specificity 74.4%, positive predictive value 82.3% and negative predictive value 97.0%. False positives occurred with a frequency of 25.6% and false negatives 1.9%. However, the cost per utility of the combined method was higher than either urine microscopic examination or urine Gram stain alone.

Urine Gram stain provided the lowest cost per utility. Economically, urine Gram stain is the proper screening tool for presumptive diagnosis of UTI.

(Source: Wiwanitkit V, Udomsantisuk N, Boonchalermvichian C. Diagnostic value and cost utility analysis for urine Gram stain and urine microscopic examination as screening tests for urinary tract infection. Urological Research 2005; 33: 220-222.)

Questions about this abstract:

1. The sensitivity of urine Gram stain was 96.2%. What does this mean and what can we conclude?
   Check suggested answer.
2. The specificity of urine Gram stain was 93.0%. What does this mean and what can we conclude?
   Check suggested answer.
3. The positive predictive value of urine Gram stain was 94.3%. What does this mean and what can we conclude?
   Check suggested answer.
4. The negative predictive value of urine Gram stain was 95.2%. What does this mean and what can we conclude?
   Check suggested answer.
5. Tests with very high sensitivity and specificity sometimes have low positive predictive value. Why is this and why is positive predictive value high in this study?
   Check suggested answer.

   The raw data for the Gram stain test can be reconstructed from the sensitivity, specificity, and positive and negative predictive values:

   Urine Gram stain	UTI		Total
   	Positive	Negative
   Positive	50	3	53
   Negative	2	40	42
   Total	52	43	95

6. Which are the false positive and false negatives and how many are there of each?
   Check suggested answer.
7. The authors say that 'False positives occurred with a frequency of 7.0% and false negatives 3.8%.' What have they done to get these numbers?
   Check suggested answer.

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Last updated: 2 March, 2007.

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