Neutrophils to lymphocyte ratio as a screening tool for swine influenza

During an outbreak of influenza like illness throat swab culture and virus isolation are done to identify causal virus, and it may cause a significant delay in diagnosis and isolation. There is a need to determine a sensitive laboratory parameter which could play a major role in identifying H1N1 infection among patients presenting with influenza like symptoms. We undertook this study to determine a laboratory parameter to identify swine influenza infection amoung patients presenting with influenza like symptoms while awaiting throat swab culture and virus isolation reports.

This retrospective study was conducted on patients presenting to A.P TB and Chest Hospital, Hyderabad, with swine influenza symptoms during September-October 2009. They were divided into swine flu positive (SW+) and swine flu negative (SW-) groups, based on their throat swab culture reports and laboratory data. Neutrophils to lymphocyte (N/L) ratio was calculated for every patient and also mean N/L ratio for two groups. Mean WBC count was also noted for both groups.

There were 55 patients in each group. The WBC count in SW+ group varied from 3000 to 10600 cells/mm 3 (mean 5714), while in SW- group it varied from 4500 to 14300 cells/mm 3 (10252). The N/L ratio was less than two for 92.7 per cent of patients in SW+ group and for 3.63 per cent patients in SW- group. N/L ratio < 2 as an indicator for swine flu had a sensitivity of 92.7 per cent and specificity of 96.36 per cent. It has a positive predictive value of 96.22 per cent and a negative predictive value of 92.8 per cent.

N/L < 2 along with a decrease in WBC count can be used as a screening tool in patients presenting with influenza like symptoms, while awaiting throat swab culture reports for confirmation.