A Study Evaluated the Accuracy of Using Midstream Urine (MSU)

A study evaluated the accuracy of using midstream urine (MSU) culture to identify 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.  Bacterial growth was determined in colony forming units per mL.  Correlation coefficients (r) were calculated for several organisms by comparing the logarithm of bacterial growth in urethral catheter specimens to that of bacterial growth in MSU specimens.  The following plots were reported: Which of the following organisms identified by MSU most likely has the lowest positive predictive value in revealing the causative pathogen for cystitis?

A) Enterococci
B) Escherichia coli
C) Group B streptococci
D) Klebsiella pneumoniae
E) Staphylococcus saprophyticus

Correct Answer:

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