ABSTRACT
Background & objectives: 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. Methods: 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. Results: There were 55 patients in each group. The WBC count in SW+ group varied from 3000 to 10600 cells/mm3 (mean 5714), while in SW- group it varied from 4500 to 14300 cells/mm3 (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. Interpretation & conclusions: 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.