656: DETERMINING THE CLINICAL UTILITY OF A PROCALCITONIN VALUE FOR PREDICTING A POSITIVE CULTURE RESULT

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Learning Objectives: Various procalcitonin (PCT) ranges have been established to guide antimicrobial therapy; however, there is no data that establish whether the initial PCT value will determine the likelihood of a positive culture result. This study aims to establish if the initial PCT value on clinical presentation can provide positive predictive value of a positive culture result.
Methods: This was a retrospective study of 813 medical intensive care unit (ICU) patients. Data points collected included patient demographics, PCT value, number of PCT assays obtained, confirmed source of infection, result of culture, ICU length of stay, and hospital length of stay. Patients were excluded if they were immunocompromised. The primary outcome of this study was to determine a PCT value that would predict any positive culture. Secondary outcomes included the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for PCT.
Results: A total of 519 patients were included to determine the impact of the initial PCT value on culture positivity. The receiver operating characteristic (ROC) result for all cultures: 0.624, pulmonary infections: 0.487, urinary tract infections: 0.428, and bacteremia: 0.779. Initial PCT values were greater for patients that had positive blood cultures compared to those patients with negative blood cultures (mean (SD), 35.4 (54.4) vs. 10.5 (31.5) [95% confidence interval (CI), -41.75 to -0.817]; P<0.001). A PCT value of 3.61 ng/ml was determined to be the threshold value for a positive blood culture result. The prevalence for bacteremia was 9.2%. For bacteremia, the sensitivity of PCT was 75%, specificity 72%, positive predictive value 20%, and negative predictive value 97%.
Conclusions: PCT was a poor predictor of culture positivity. An initial PCT value less than 3.61 ng/ml may be useful in predicting whether a bacteremia is absent. PCT should not independently be used to determine initiation of antibiotic therapy.
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