2 false-positive dengue cases turn out to be COVID-19-positive in SG

Two patients in Singapore who initially tested positive for dengue using rapid serological testing turned out to be COVID-19 cases, a recent case report has shown.

“Dengue and COVID-19 are difficult to distinguish because they have shared clinical and laboratory features,” according to the researchers led by Dr Gabriel Yan from National University Health System, Singapore.

“Failing to consider COVID-19 because of a positive dengue rapid test result has serious implications not only for the patient but also for public health,” they highlighted.

The first case described was a 57-year-old man with no red flags in terms of medical, travel, and contact history. Upon presentation to a regional hospital, he reported having fever and cough already for 3 days. Blood count showed thrombocytopenia, though chest radiography appeared normal. After testing negative in a rapid serological test for dengue IgM, IgG, and NS1*, he was discharged. [Lancet Infect Dis 2020;doi:10.1016/S1473-3099(20)30158-4]   

However, his fever persisted, along with worsening thrombocytopenia and new-onset lymphopenia upon a subsequent visit to a primary healthcare clinic. A repeat dengue rapid test again turned out positive for dengue IgG and IgM. He was hence referred to a hospital for dengue with worsening dyspnoea and cough.

His ordeal took a turn when signs from another chest radiography prompted testing for SARS-CoV-2, which turned out positive by RT-PCR of a nasopharyngeal swab sample. Using the original sample which initially tested positive on rapid serological testing, the results came out negative for dengue when run on RT-PCR. Additional blood and urine samples also showed negative results on RT-PCR.

Furthermore, the patient tested negative when he underwent a repeat dengue rapid test. Therefore, the initial result was considered a false positive.

The second case of a 57-year-old woman showed a similar saga — presenting with fever, muscle pain, and mild cough of 4 days and diarrhoea of 2 days. Thrombocytopenia was present, with positive serological results on rapid dengue test for IgM. She had no relevant medical, contact or travel history, and was discharged as an outpatient pending follow-up for dengue fever.  

After 2 days, she returned with persistent fever, worsening thrombocytopenia, and new-onset lymphopenia. Liver function tests showed abnormal ALT, AST**, and bilirubin levels; while chest radiography was normal. Hospitalization for dengue fever ensued.

Three days after admission, she developed dyspnoea and remained febrile. Repeat serological dengue test turned out negative, as was dengue test by RT-PCR. Further RT-PCR of nasopharyngeal swab confirmed SARS-CoV-2 infection.

“Our cases highlight the importance of recognising false-positive dengue serology results (with different commercially available assays) in patients with COVID-19,” said Yan and co-authors.

“We emphasize the urgent need for rapid, sensitive, and accessible diagnostic tests for SARS-CoV-2, which need to be highly accurate to protect public health,” they stated.