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?

Question

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?

Quizplus · Accepted Answer

Explanation: 11ec2b69_bde9_0cde_8f63_c1696f603690_MD0005_00 The plots represent the correlation between bacterial counts in urethral catheter samples and midstream urine (MSU) samples. The units used for the x-axis and y-axis are in the base-10 logarithmic scale (log₁₀); therefore, the log₁₀ quantities 1-5 represent 10¹-10⁵ colony forming units (CFU) per mL. The number of squares on each plot reflects the number of specimens in which organism growth was detected. For example, the Escherichia coli plot has the largest number of squares, reflecting this organism's predominance among the samples tested. The coordinates of the squares reflect the different growth patterns in the urethral catheter and MSU samples. For example, enterococci and group B streptococci mostly grew in the MSU specimens but not in the urethral catheter specimens (gold standard to diagnose cystitis), suggesting that these organisms are rarely associated with cystitis. The correlation coefficients for these organisms were also small (eg, 0.341 for enterococci). plots Correlation coefficient (r) values range from −1 to +1. The operational signs indicate a positive or negative direction of association between 2 variables; the null value is 0 and denotes no association. The closer the r value is to its margins [−1, +1], the stronger the association. In this scenario, the lowest r value (0.341) was for enterococci. This means that a positive result for enterococcal growth based on MSU culture is only weakly associated with a positive result based on urethral culture (gold standard). Therefore, the positive result for enterococci on MSU has a low predictive value for cystitis. (Choices B, C, D, and E) All of these have higher (more predictive) positive r values than enterococci. Determining the positive predictive value by counting the points on the graphs is suboptimal because no specific cutoff for bacterial growth (CFU/mL) is provided to define cystitis and because each point could represent >1 specimen. Educational objective: Correlation coefficient (r) values indicate a positive or negative direction of association between 2 variables. The closer the r value is to its margins [−1, +1], the stronger the association.

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