A fast and cost-effective new dipstick test for Zika virus (ZKV) can distinguish between it and dengue viruses (DENV) without cross-reactivity, according to a report in Science Translational Medicine. The diagnostic also is an advance, said the developers, because it can detect viral proteins at far lower concentrations than commercially available assays (4 to 21 ng/mL versus 75 to 150 ng/mL).
Because ZKV and DENV have homologous proteins and nucleic acids, molecular, antigenic and serologic diagnostic tests for them can be associated with cross-reactions and false-positive results. The developers of the new test, Bosch and colleagues, report that the strips showed serotype-specific signals that were statistically distinct (P < 0.001). Images of the strips were captured with a mobile phone camera and automated image recognition and processing was used to objectively quantify the authors’ data.
To develop the dipstick, the researchers characterized monoclonal antibody pairs and translated them into rapid immunochromatography tests that would specifically detect the viral nonstructural 1 (NS1) protein antigen and distinguish the 4 DENV serotypes (DENV1-4) and ZIKV without cross-reaction. The immunochromatography tests were validated in de-identified serum samples of patients from Brazil, Mexico, Colombia, Panama, Guatemala, and India. The samples had been drawn from febrile patients during routine care and frozen. Before the new tests were used on the samples, the sera were validated with reverse transcriptase polymerase chain reaction (RT-PCR), and ELISA was also used on positive tests.
Using a 30-µL serum sample, the sensitivity and specificity of the DENV1-4and the pan-DENV test, which detects all 4 dengue serotypes, ranged from 0.76 to 1.00. Sensitivity and specificity of the ZIKV rapid test were 0.81 and 0.86, respectively, using a 150-µL serum input. Serum ZIKV NS-1 protein concentrations were about 10-fold lower than corresponding DENV NS1 concentrations in infected patients; moreover, ZIKV NS1 protein was not detected in PCR-positive patient urine samples. The rapid tests detected DENV NS1 at 2 to 5 days after onset of symptoms and the ZIKV NS1 rapid test detected ZIKV NS1 at days 2 to 8.
The researchers said the current cost for each test strip is $5, largely because they involve use of commercial gold nanoparticles and small-scale antibody production. They believe the cost of the nanoparticles can be “decreased by 1000-fold by using laboratory-made nanoparticles that have excellent performance” and say that cost will be lowered further by scale-up of antibody production.
The authors noted that the testing they have described is effective only for analysis of samples collected during the acute phase of virus infection, before the virus is cleared by the immune system. Their approach and reagents, they said, “have immediate application in differential clinical diagnosis of acute ZIKV and DENV cases, and the platform can be applied toward developing rapid antigen diagnostics for emerging viruses.”