Differentiating secondary from primary dengue using IgG to IgM ratio in early dengue: an observational hospital based clinico-serological study from North India

Background: Secondary dengue causes more severe disease than the primary. Early on, it is important to differentiate the two. We tried to find important clinical and laboratory differences between the two for the purpose of early differentiation. Methods: One hundred fourteen patients confirmed on reverse transcriptase-polymerase chain reaction (RT PCR) were studied. On day 2 of illness IgM and IgG indices were studied for calculation of IgG/IgM ratio. A one-step immunochromatographic assay was used for classification of patients into primary and secondary dengue. Patient characteristics were also studied. Results: Dengue serotype 1 was the most common found in 60.5% patients. 66.7% (76 patients) had secondary dengue. Secondary dengue cases had a higher mean temperature (101.56 +/- 1.55 vs. 100.79 +/- 1.25, degrees F, p 0.015), lower platelet counts (50.51 +/- 38.91 vs. 100.45 +/- 38.66, x 10(3)/micl, p < 0.0001) and a significantly higher percentage of Dengue hemorrhagic fever/Dengue shock syndrome (38.2% vs. 2.6%, p < 0.0001). In early phase of dengue NS1 and PCR were found to be better tests for diagnosis and later IgM is better. The IgG/IgM ratio of = 1.10 had a sensitivity of 100%, specificity of 97.4% and accuracy of 67.5% in differentiating secondary from primary dengue. Conclusion: Early on in the clinical course, IgG/IgM ratio can play an important role to differentiate the two. We found the ratio of >= 1.10 to be the best cut off for the same.