ROLE OF THE COMPLETE BLOOD-COUNT IN DETECTING OCCULT FOCAL BACTERIAL-INFECTION IN THE YOUNG FEBRILE CHILD

Previous studies of the value of the complete blood count (CBC) in dis tinguishing viral from bacterial infection in young febrile children h ave failed to exclude children with clinically evident bacterial infec tion and thus have inflated the positive predictive value of the test for occult focal infection. We prospectively studied 2492 children 3-2 4 months of age who presented to a children's hospital emergency depar tment between March 1989 and August 1990 with fever (greater-than-or-e qual-to 38.0-degrees-C) of acute (less-than-or-equal-to 4 days) onset but no evident bacterial focus of infection, 433 (17.4%) of whom recei ved a CBC. We also carried out an 8-year retrospective analysis to est imate prior, or pre-test, probabilities (prevalences) and examine CBC results for rare occult bacterial infections (meningitis, osteomyeliti s, and septic arthritis). Estimated prior probabilities for the four m ost common categories of infection that can be diagnosed at the initia l visit were: non-pneumonitic viral infection, 88.6% in boys and 86.0% in girls; pneumonia, 8.5% in both sexes; urinary tract infection (UTI ), 3.0% in boys and 5.5% in girls; and bacterial meningitis, 0.0066% i n both sexes. The likelihood (sensitivity) of a total white blood cell (WBC) count greater-than-or-equal-to 15,000/mm3 was 25.5, 64.5, 62.5, and 50.0% for viral infection, pneumonia, UTI, and meningitis, respec tively. Among children with a high total white blood cell count, neith er a total polymorphonuclear count greater-than-or-equal-to 10,000/mm3 nor a band count greater-than-or-equal-to 500/mm3 was associated with significantly elevated likelihoods for occult pneumonia or UTI, a fin ding confirmed by multiple logistic regression analysis. Based on the prior probabilities and likelihoods of WBC greater-than-or-equal-to 15 ,000/mm3 for each diagnosis, the posterior probabilities (positive pre dictive values) were: viral infection, 75.3% in boys and 70.8% in girl s; pneumonia, 18.5% and 17.9%; UTI, 6.2% and 11.3%; and bacterial meni ngitis, 0.01% in both sexes. Thus a high WBC count approximately doubl es the overall probability of occult focal bacterial infection. Even i n the face of a high WBC count and ''shift to the left'', however, a v iral diagnosis remains far more likely. Given the rarity of occult bac terial meningitis, the apparent lack of morbidity associated with gros s underdetection of occult pneumonia, and the existence of a less inva sive, more specific test (the urinalysis) for the diagnosis of UTI, cl inicians should critically re-examine their use of the CBC in the youn g febrile child.