Dengue fever (DF) is a serious illness in many tropical and subtropical countries. In ancient Chinese literature it has been referred to as “water poison” due to its association with flying insects. The etiological organisms responsible are dengue viruses (DENV) belonging to the family Flaviviridae, genus
Flavivirus. Dengue virus has 4 serotypes : 1,2,3,4. The WHO 2009 classification divides dengue depending on the severity of illness. It classifies dengue as with or without warning signs (pain abdomen, severe vomiting, fluid retention and third spacing, mucosal hemorrhage, malaise and drowsiness, hepatomegaly, high hematocrit with low platelets); and severe dengue (severe plasma leakage, severe bleeding, or organ failure) [
2]. On the other hand 1997 classification divides it into undifferentiated fever, dengue fever (DF), and dengue haemorrhagic fever (DHF) [
3]. We chose the latter definition in our study to allow for comparison with previous studies most of which have used the same definition. In India, Dengue has been known since 1946 and has been present in different parts of the country as epidemics and outbreaks. In 1960s it had established its endemicity in India and different strains have been replacing the older ones ever since [
4]. Asymptomatic infections are much more frequent than symptomatic ones [
5]. Severe DENV infection like dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) are chiefly seen with secondary infection [
6,
7]. Considering this, it becomes very important to distinguish primary from secondary dengue early on in the course of illness. It helps in predicting prognosis and also in deciding about whether a patient needs admission and close monitoring or could be managed at home. This becomes more important during epidemics and outbreaks when hospitals are flooded with patients and early triage becomes necessary.
The Hemagglutination inhibition (HI) test was the standard reference test recommended by the World Health Organization (WHO) to classify primary and secondary dengue virus infection [
8]. This reference test requires paired serum samples and cannot give an early diagnosis, exhibits high cross-reactivity and requires chemical pre-treatment to remove nonspecific inhibitors of hemagglutination [
9,
10]. So, researchers have tried to find out better and easier methods to differentiate primary from secondary dengue. IgG avidity can be used to discriminate among primary dengue and secondary dengue infection [
11].
The aim of this study is to differentiate primary from secondary dengue using IgG to IgM ratio from enzyme-linked immunosorbent assays (ELISA) based detection of specific antibodies. The IgG and IgM ELISA has the benefit of easier use, being suitable for surveillance and for large-scale studies. Several studies have been performed to discriminate primary and secondary DENV infection using the ratio of IgG and IgM at the different days of onset of symptoms [
12‐
16]. We look for the best IgG/IgM ratio to differentiate primary and secondary DENV infection in our population within the first three days of symptom onset.