OBJECTIVES. The objectives of the study were (1) to study the test performance of procalcitonin for identifying serious bacterial infections in febrile infants ≤90 days of age without an identifiable bacterial source and (2) to determine an optimal cutoff value to identify infants at low risk for serious bacterial infections.

METHODS. A prospective observational study was performed with febrile infants ≤90 days of age presenting to an urban, pediatric, emergency department. Serum procalcitonin levels were measured by using an automated high-sensitivity assay. An optimal procalcitonin cutoff value was selected to maximize sensitivity and negative predictive value for the detection of serious bacterial infections. Infants were classified as having definite, possible, or no serious bacterial infections.

RESULTS. A total of 234 infants (median age: 51 days) were studied. Thirty infants (12.8%) had definite serious bacterial infections (bacteremia: n = 4; bacteremia with urinary tract infections: n = 2; urinary tract infections: n = 24), and 12 infants (5.1%) had possible serious bacterial infections (pneumonia: n = 5; urinary tract infections: n = 7). Mean procalcitonin levels for definite serious bacterial infections (2.21 ± 3.9 ng/mL) and definite plus possible serious bacterial infections (2.48 ± 4.6 ng/mL) were significantly higher than that for no serious bacterial infection (0.38 ± 1.0 ng/mL). The area under the receiver operating characteristic curve was 0.82 for definite serious bacterial infections and 0.76 for definite and possible serious bacterial infections. For identifying definite and possible serious bacterial infections, a cutoff value of 0.12 ng/mL had sensitivity of 95.2%, specificity of 25.5%, negative predictive value of 96.1%, and negative likelihood ratio of 0.19; all cases of bacteremia were identified accurately with this cutoff value.

CONCLUSIONS. Procalcitonin has favorable test characteristics for detecting serious bacterial infections in young febrile infants. Procalcitonin measurements performed especially well in detecting the most serious occult infections.

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