A 32-year-old woman comes to the physician with urinary burning and frequency. She has no fever or back pain. Midstream urine (MSU) cultures grow 10⁵ colony forming units (CFU)/mL of enterococci. A previous publication studied the accuracy of MSU culture in identifying the causative pathogen in patients with uncomplicated cystitis. Women ages 18-49 with symptoms of cystitis provided MSU specimens for culture. The investigators also collected urine by means of a urethral catheter for culture, which served as the gold standard for confirming the causative pathogen. The following results relating to enterococcal growth were reported for 404 specimens: Based on the study results, what is the positive predictive value of enterococcal growth of 10⁵ CFU/mL in the MSU sample?

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Explanation: The positive predictive value (PPV) is the probability that an individual truly has a disease given a positive test result. The formula is: PPV = TP / (TP + FP), where TP denotes true positives and FP denotes false positives. To calculate PPV, a contingency (2x2) table can be constructed to find TP and FP first. This can be done using the data provided on sensitivity and specificity of midstream urine culture (test whose diagnostic performance is being evaluated) compared to urethral catheter culture (gold standard to confirm or exclude cystitis). A standard contingency table will have the format shown below: 11ec2b69_c104_561c_8f63_7d1a5a0ae246_MD0005_00 Sensitivity = TP / (TP + FN); this is the probability of a diseased person testing positive (FN denotes false negatives). In this example, the sensitivity for enterococcal growth >10⁵ CFU/mL is given as 2/4, so sensitivity = TP / (TP + FN) = 2/4. This means that TP = 2 and (TP + FN) = 4; therefore, TP = 2 and FN = 4 - 2 = 2. Specificity = TN / (TN + FP); this is the probability of a non-diseased person testing negative (TN denotes true negatives). In this example, the specificity for enterococcal growth of >10⁵ CFU/mL is given as 394/400, so specificity = TN / (TN + FP) = 394/400. This means that TN = 394 and (TN + FP) = 400; therefore, TN = 394 and FP = 400 - 394 = 6. Based on this information, the following contingency (2x2) table can be constructed: Therefore, PPV = TP / (TP + FP) = 2 / (2 + 6) = 2/8 = 1/4 = 25%. Educational objective: The positive predictive value (PPV) of a diagnostic test is the proportion of patients with a positive test result who actually have the disease. The formula is PPV = TP / (TP + FP), where TP denotes true positives and FP denotes false positives. PPV can be calculated from a contingency table. References: Understanding and using sensitivity, specificity and predictive values. (http://www.ncbi.nlm.nih.gov/pubmed/18158403)

A 32-Year-Old Woman Comes to the Physician with Urinary Burning