The role of readily available clinical, laboratory and radiologic findings in distinguishing a/H1N1/2009 influenza from other causes of acute febrile respiratory illness under pandemic conditions.

BACKGROUND: During an influenza pandemic, clinicians need easily available clinical and laboratory criteria to distinguish influenza from similar respiratory illnesses. We compared A/H1N1/2009-polymerase chain reaction (PCR)-positive and matched PCR-negative hospitalized patients with suspected H1N1 influenza to identify factors that could assist physicians at patient admission.

OBJECTIVES: To identify factors significantly associated with A/ H1N1/2009 infection.

METHODS: A group of 145 patients with PCR-confirmed A/H1N1 2009 influenza admitted between 27 May 2009 and 3 December 2009 was matched with 145 PCR-negative patients by age, epidemiological week and pregnancy status. Epidemiological and clinical parameters and radiological findings on initial chest X-ray were compared between the two groups.

RESULTS: Asthma (PCR+ 26%, PCR- 12%, P = 0.006) and military service (PCR+ 13%, PCR- 4%, P = 0.15) were associated with PCR-positive status in non-pregnant patients. At presentation, fever, cough, myalgia and fulfilling the pandemic influenza case definition were significantly more frequent in nonpregnant PCR+ patients (62/90/43/59% in PCR+ versus 38/69/30/35% in PCR-). In pregnant patients, fever and fulfilling the case definition were significantly associated with PCR-positive status. Mean leukocyte and absolute lymphocyte counts were significantly lower in both pregnant and nonpregnant PCR-positive patients. Significantly more PCR-negative non-pregnant patients (43% vs. 22% PCR+, P = 0.004) had abnormal chest X-ray (CXR) findings on presentation. In PCR-positive patients, patchy consolidation and interstitial infiltrates were the most common abnormalities.

CONCLUSIONS: Under the conditions generated by the A/ H1 1/2009 pandemic, radiological findings did not distinguish reliably between influenza and other febrile respiratory illnesses. Asthma, military service, the pandemic case definition (particularly fever, cough and myalgia)