Diagnosing influenza: the value of clinical clues and laboratory tests

Our goal was to determine the utility of clinical clues, white blood cell (WBC) and differential counts, and a rapid antigen test for differentiating influenza from coexistent infectious diseases during influenza epidemics.

Data were collected during 3 consecutive influenza outbreaks over a 2-year period. The information collected included date of onset, symptoms, vaccine status, WBC and differential counts, ZstatFlu test (ZymeTx, Oklahoma City, Ok), and influenza culture. Using culture positivity as the criterion for influenza diagnosis, we compared cases with noncases on each variable independently and by logistic regression. Receiver operating characteristic curves were plotted for WBC count, ZstatFlu, and their combination in an effort to determine the most useful diagnostic strategy.

We included consecutive patients presenting to a family practice office with fever, cough, sore throat, myalgia, and/or headache during flu season.

The outcomes were sensitivity, specificity, and other measures of test accuracy.

Culture-positive cases could not be reliably distinguished from those that were culture negative using symptoms or vaccination status. Both WBC count and ZstatFlu results discriminated fairly well, and their combination did somewhat better. Differential counts were not helpful. WBC counts above 8000 were associated with a low probability of influenza. The sensitivity and specificity of the ZstatFlu were 65% and 83%, respectively.

Our data suggest that symptoms and vaccine status do not reliably identify patients with influenza. Use of WBC counts and the ZstatFlu test can be helpful. The sequence, combination, and criteria for use of these tests depend on tradeoffs between undertreatment of influenza cases and the overtreatment of noninfluenza cases, and the cost and benefit projections for individual patients.