Rapid diagnostic tests show great promise, but also unacceptable inconsistency

Rabies diagnostics tests that are cheap and easy to use could hugely benefit rabies control efforts, providing confirmation of animal cases to help save lives and refine control strategies to better protect communities. The gold standard test, the Fluorescent Antibody test (FAT) requires laboratory equipment, trained staff, and temperature-sensitive reagents, limiting their application. In contrast, recently developed rapid immunodiagnostic tests for rabies diagnosis promise testing outside the laboratory, require virtually no equipment and minimal training, and produce a result in minutes. Such tests are available on the market now, but do they work? Two recently published papers demonstrate both the promise and the current inadequacies of these tests.

A study, published in PLOS Neglected Tropical Diseases this month, evaluated the Anigen Rapid Rabies Test® with 48 samples in a field laboratory (in N’Djamena, Chad) and 73 samples in an international reference laboratory (Institute Pasteur, Paris, France). In the reference laboratory in France, the test produced a specificity of 93.3% and sensitivity of 95.3% compared to the gold standard FAT test. In the field laboratory, the rapid test yielded a higher reliability than the FAT test (though here the FAT did not follow the gold standard methodology exactly) particularly on fresh and decomposed samples. The study concluded that the test performed excellently in term of both ease of use and the reliability of the results. Importantly, these authors omitted a dilution step from the manufacturer’s protocol.

However, a second study, published in the same journal in June tested six commercially available rapid rabies tests (including the Anigen Rapid Rabies Test®) in a controlled laboratory setting. Over 80 (naturally and lab-infected) animal brain samples were tested at the Friedrich-Loeffler-Institute in Germany, a WHO Collaborating Centre for Rabies Surveillance and Research and 20 naturally-infected animal brain samples were tested at the OIE Rabies Reference Laboratory, in Onderstepoort Veterinary Institute, South Africa. Each brain sample was tested by each rapid test kit and also by the gold standard FAT.

The second study concluded that the tests did not give reproducibly accurate results. While all six tests showed good specificity (i.e., they did not come up with “false positives”), but none demonstrated good sensitivity across the different sample sets. Overall, more than half of all positive samples (i.e., those with positive FAT and PCR results) yielded “false negative” LFD results. Some LFD tests did better than others, and in some cases specific batches of the same test did better than others, but overall the results were disappointing.

These authors suggest that commercially available rabies LFDs cannot be recommended for routine diagnosis and surveillance at this time.  However, acknowledging the great potential for these tests, they also state that the study “is not meant to discredit the use of LFDs for rabies diagnosis but rather to encourage producers to substantially improve and assure the quality of their products”.

Both studies demonstrated the ability to recover virus RNA from the test strips for genetic analysis by PCR, which means that the tests could be used as a vehicle to ship viral RNA to reference laboratories for further laboratory confirmation of the diagnosis and for genetically characterising the viruses, which could be extremely valuable to epidemiological investigations.

The authors of the first study suggest that their omission of the dilution step could have contributed to their more consistent results, but clearly more assessment of these tests’ reliability and refinement of their instructions for use will be needed before they can fulfil their potential. Further validation is currently ongoing in pilot field studies in Chad, Mali and Ivory Coast and is expected to provide a large data set over the next two years. 

Written by Louise Taylor, based on Lechenne et al (2016) “Validation of a Rapid Rabies Diagnostic Tool for Field Surveillance in Developing Countries” and Eggerbauer et al (2016) “Evaluation of Six Commercially Available Rapid Immunochromatographic Tests for the Diagnosis of Rabies in Brain Material” both published in PLOS NTDs.