Dengue fever is mostly a rather undifferentiated febrile disease with non-specific signs and symptoms, and molecular and serological tests are can be used to confirm the clinical diagnosis.
Serological confirmation of dengue has become available to many laboratories by commercially available assays. Dengue serology is applied in different settings, such as for surveillance, in health care facilities in endemic areas and in travel clinics in non-endemic areas The applicability and quality of serological tests in dengue endemic regions has to be judged against a background of potential cross reactivity with other flavi-viruses, difficulties in distinguishing primary from secondary infections and technological problems related to the fact that most dengue endemic regions are relatively poor of resources.
Enzyme linked immuno-assay (ELISA) is a convenient technique which allows laboratories to test numerous samples in a short time. Different assays are available on commercial basis. Dengue IgM capture ELISA (MAC-ELISA) and IgG ELISA are both sensitive and specific assays for detection of dengue antibodies but distinction from other endemic flavivirusses is important, in Southeast Asia especially Japanese encephalitis B (JEB) virus[2,3]
Assays that apply antigen from dengue virus type (DEN) 1 through 4, have a high sensitivity and specificity, typically ranging from approximately 90 to 100 %, but do not discriminate between the four serotypes[3,4] The distinction between primary and secondary infections is now mainly based on recognizing the different IgM and IgG responses to primary and secondary infections in two samples taken from a febrile patient in the acute stage of disease and after convalescence[5,6]
Often however, the sample cannot be tested at the spot and needs to be stored and transported before analysis. Blood spots on filter paper are often used as an alternative to collecting serum samples. Dried in the air, they can be stored easily. Filter papers are used for several purposes, such as screening of newborns for congenital hypothyroidism and phenylketonuria (PKU), DNA diagnostics and detection of antibodies[7,8] They are also used for ELISA detection of antibodies against dengue and other viral infections. However, although several technical aspects have been studied, including the duration and temperature of storage, experience is limited[6,9-13]
In this study we investigated several aspects of the variability in dengue IgM and IgG ELISA results in serum and blood spots on filter paper, and their decay during storage, from febrile patients who presented at primary health care facilities in southern Vietnam, an area with a high incidence of dengue virus infections which is also endemic to JEB virus.