Dengue fever (DF) is an arthropod borne viral diseasecaused by mosquito borne single stranded RNA positive strand Dengue virus (DENV) of the family Flaviviridae, genus Flavivirus [1, 2]. The disease is transmitted by the female Aedes aegypti and Aedes albopictus mosquito which transmit the virus during the blood meal. In recent years, DF has become a major international health problem affecting tropical and sub-tropical regions around the world - especially urban and peri-urban areas [3, 4]. The global annual incidence is about 390 million dengue infections per year, of which 96 million manifests clinically [5]. It presents with a diverse clinical picture that ranges from asymptomatic illness to DF to the severe illness of dengue hemo-rrhagic fever (DHF)/ dengue shock syndrome (DSS).

The preliminary diagnosis relies on the clinical symptoms. The confirmation of the diagnosis is routinely done by serological tests. The methods include detection of virus by cell culture and immunofluorescence, detection of virus antigen by Enzyme Linked Immunosorbent Assay (ELISA), detection of anti dengue virus antibody by haem-agglutination inhibition (HI), complement fixation test (CF), neutralization test, ELISA and detection of virus nucleic acid by Real Time Reverse Transcription Polymerase Chain Reaction (RT-PCR) [6,7].

To diagnose the acute infection, the preferred method used these days is detection of Non Structural 1(NS1) protein and IgM and IgG anti dengue virus antibody using in vitro immunochromatographic assay. NS1 is a highly conserved glycoprotein which is present in high concentrations in the serum of dengue-infected patients from Day 1 to Day 9 after the onset of fever [6,7]. In case of primary dengue infection, IgM becomes detectable on Day 3 to 5 of illness and persist for 2 to 3 months, whereas IgG appear by the fourteenth day and persist for life. Therefore, patients with secondary infections will have a positive IgG result, usually, but not always with a positive IgM result [8, 9]. There is alteration in various hematological parameters during dengue fever. Leuopenia, thrombocytopenia and presence of atypical lymphocytes are commonly observed in dengue cases. Raised hematocrit (Hct) is a known finding in dengue hemorrhagic fever [10, 11].

(MCH), Mean Corpuscular Hemoglobin Concentration (MCHC), Red Cell Distribution Width-Coefficient of Variation (RDW-CV), Total leukocyte count (TLC), Differential leucocyte count (DLC), Platelet Count (P/C), Platelet Distribution Width (PDW), Mean Platelet Volume (MPV), Platelet-Large Cell Ratio (P-LCR) and Plateletcrit (PCT). Mean value of each parameter was calculated using SPSS software. Annova test was used to calculate the p value to find correlation between hematological and serological parameters

Amongst 2022 cases, majority (83.13%) were seen in the year 2016 followed by 16.71% cases in the year 2017. During the year 2016 and 2017 maximum number of cases were seen in the month of September i.e. 56.04% and 44.37% respectively. Teenagers and young adults are most commonly affected as 30.8% cases were in the age group of 11-20 years. Least number of cases were seen in elderly (1.6%). There was male predominance (M:F- 1.8:1). The distribution of cases into 6 serogroups is shown in Table 1.

Table-1: Groups of serum markers

Evaluation of various hematological parameters (RBC Parameters, WBC Parameters, Platelet Parameters) is shown in Table 2.

Table-2: Evaluation of hematological parameters

Dengue fever is a growing public health concern in most tropical countries. To decrease the dengue related mortality and morbidity, it is very important to know the various hematological parameters that are altered during the illness. It is equally important to know the various serological parameters and their interpretation.

Over the past few years, there has been a decline in the number of dengue cases probably due to the increasing awareness amongst the people with the help of various health programs and campaigns in media as well as change in their health seeking behavior.

The lower infection rates in females of Asian community could also be due to the fact that many of them remain at home and are less exposed to this vector borne infection [12]. Also, the immune response in females is more competent than in males, resulting in greater production of cytokines, thus providing protection [13]. Peripheral blood parameters (Hemogram) revealed a wide range of variation from normal to decrease or increase when compared with the normal reference range.

Hemoglobin values were normal in majority of the cases and mean was 13.30 g/dl in females and 13.75 g/dl in males. Anaemia was seen in only 25.4% of the cases. Low Hb levels can be due to Disseminated Intravascular Coagulation (DIC), where chemicals responsible for clotting are used up and lead to severe bleeding [14].

At the same time, significantly higher level of hemoglobin were seen in the acute phase due to fluid leakage leading to concentration of plasma, causing a rise in hemoglobin [15]. Plasma leakage occurs due to cytokine release which is manifested as haemoconcentration and is seen as increase in hematocrit 20% above average for age, sex and population. It should be remembered that, with the progression to DHF, there is a 20% increase in hematocrit from the baseline along with presence of thrombocytopenia [16, 17]. In the present study, raised hematocrit was seen only in 5.8% of the cases. It was normal in 68% cases and mean hematocrit values were 40.78% in females and 41.59% in males. RDW-CV and RBC indices were normal in majority of the subjects. MCH and MCHC had a mean value of 28.66 pg and 33.06 g/dl respectively which was in agreement with Advani et al [18].

Platelets are involved in hemostasis, tissue repairing and infection. They are the natural sources of various growth factors like Platelet Derived Growth Factor (PDGF), Vascular Endothelial Growth Factor (VEGF), Insulin Like Growth Factor-1 (IGF-1) and Transforming Growth Factor β (TGF-β). These growth factors have an important role in inflammation, angiogenesis, repair and regeneration of tissues [26]. Activation of platelets causes morpho-logical alterations in them. Thrombocytopenia is a common finding in dengue patients [18, 20, 22, 24]. It was noted in 62.6% cases in the present study.The mean value of platelet count was also reduced (129.89 X 10³/µl) in this study. It is attributed to the direct bone marrow suppression by the virus, anti dengue antibody- mediated platelet destruction, peripheral consumption of platelets and isolated viral replication in the platelet.

The release of high levels of platelet activating factor may induce platelet consumption and augment adhesiveness of vascular endothelial cells resulting in thrombocytopenia [27, 28]. A notable decrease in platelet count with disease progression and in secondary dengue infection was observed in the present study emphasizing the need to regularly monitor the platelet count during the illness to avoid bleeding complications. Upon activation, the platelets exhibit morphological changes. There is an increase in the number of pseudopodia and variation in their sizes leading to alterations in the platelet distribution width (PDW) [29]. In the present study, only 43.1 % of the cases had increased PDW and the mean value of PDW was 15.68fL in females and 14.99fL in males. MPV acts as a surrogate marker of bone marrow activity. A high MPV indicates increased megakaryocyte activity where as a low MPV indicates marrow suppression and increased risk of bleeding [30].

In the present study, MPV values were normal in 60% of the cases with a mean value of 12.10fL in females and 11.76fL in males. Platelet large cell ratio is defined as the percentage of platelets that exceed the normal volume of 12 fL in the total platelet count [31].

Approximately 65% of the cases had normal P-LCR in the present study with a mean value of 40.03% in females and 38.92% in males. Plateletcrit, a measure of total platelet mass and an effective screening tool for detecting platelet quantitative abnormalities was reduced in majority of the subjects (65.6%) and was normal in 32.7%.