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Observations on false positive reactions in the rapid NOW Filariasis card test
Paul E Simonsen1, Stephen M Magesa
1Danish Bilharziasis Laboratory, Charlottenlund, Denmark. pes@bilharziasis.dk
Insights
The NOW Filariasis card test accurately detects Wuchereria bancrofti infection when read at 10 minutes. Later readings may show false positives, but experienced users can distinguish true results by line appearance.
Area of Science:
- Tropical Medicine
- Parasitology
- Diagnostic Testing
Background:
- Wuchereria bancrofti infection diagnosis relies on rapid field tests.
- Concerns exist regarding false positive results with prolonged reading times for the NOW Filariasis card test.
Purpose of the Study:
- To investigate the phenomenon of false positive reactions in the NOW Filariasis card test.
- To determine if true and false positive results can be differentiated after prolonged reading times.
Main Methods:
- A survey was conducted in a Tanzanian village endemic for filariasis.
- NOW Filariasis card test results were compared with TropBio ELISA for circulating filarial antigen (CFA).
- Visual inspection of test lines (shape and color) was used to differentiate true and false positives.
Main Results:
- Individuals testing positive at 10 minutes were confirmed positive by ELISA.
- Test cards negative at 10 minutes but positive later showed false positives when compared to ELISA.
- Distinct visual characteristics (shape and color) differentiate true and false positive lines.
Conclusions:
- The NOW Filariasis card test is reliable for Wuchereria bancrofti diagnosis when read at the recommended 10-minute interval.
- Experienced users may still accurately interpret results even with later readings by observing line characteristics.
- Adherence to the 10-minute reading time is crucial to avoid false positives in filariasis diagnosis.
Abstract:
The NOW Filariasis card test is a useful tool for rapid field diagnosis of Wuchereria bancrofti infection, based on detection of specific circulating filarial antigen (CFA) in the patients' blood. Concern has been raised that a high proportion of infection negative individuals develop false positive reactions in these tests when the test cards are left for a prolonged period before being examined. We carried out a survey in an endemic Tanzanian village to investigate this phenomenon. Individuals who were positive in the NOW Filariasis test at 10 min after specimen application were also positive in the TropBio ELISA for CFA, and thus appeared to be truly positive. Many of the test cards that were negative at 10 min developed a positive line later, but these lines appeared to be falsely positive when the TropBio test was used as the gold standard. Close examination revealed that true and false positivity lines could be distinguished on their shape and colour. The study thus reaffirmed that test cards should be read after 10 min to avoid false positives, but it also indicated that experienced test card users should be able to make a correct diagnosis even at a later time.
