Prototype Test Developed for Ocular Trachoma

An evaluation has been made of a prototype point-of-care (POC) test, developed for field diagnosis of ocular Chlamydia trachomatis, in low prevalence settings.

The clinical signs of active trachoma are often present in the absence of ocular C. trachomatis infection in low prevalence and mass treated settings and treatment decisions are currently based on the prevalence of clinical signs.

A simple point-of-care (POC) test has been compared in laboratory testing for C. trachomatis infection to a polymerase chain reaction (PCR) which is highly sensitive but expensive and requires well-trained staff. Scientists from the London School of Hygiene and Tropical Medicine, (London, UK), working with others in the Gambia and Sengal screened children under the age of 10 for signs of trachoma. Two ocular swabs were taken from the right eye. The first swab was tested by the POC test in the field. This assay detects the chlamydial lipopolysaccharide (LPS), using lateral flow technology and this assay was designed specifically for use in resource-limited settings, and therefore has no electricity, water, or laboratory equipment requirements. The second swab was tested for the presence of C. trachomatis by Amplicor PCR.

A total of 3,734 children were screened, 950 in the first and 1,171 in the second Gambian study, and 1,613 in Senegal. The sensitivity of the prototype POC test (Diagnostics Development Unit, University of Cambridge, UK), ranged between 33.3% and 67.9%, the specificity was between 92.4% and 99.0%. The positive predictive value ranged between 4.3% and 21.0%, and the negative predictive value was between 98.0% and 99.8%. The rate of false-positives increased markedly at temperatures above 31.4 °C and relative humidities below 11.4%.

Although the POC test is a qualitative assay, the signal strength was scored on a scale from a weak 0.5 to a strong 5.0 in the field. There was a significant association between increased organism load and increased POC test signal strength. The Amplicor Chlamydia trachomatis PCR assay is a product of Roche Diagnostics (Burgess Hill, UK).

The authors concluded that the in its present format, this prototype POC test is not suitable for field diagnosis of ocular C. trachomatis as its specificity decreases in hot and dry conditions. In the absence of a suitable test for infection, trachoma diagnosis remains dependent on clinical signs and this will likely result in the unnecessary distribution of mass antibiotic treatment. The study was published on in the online journal Public Library of Science Neglected Tropical Diseases.

Related Links:
London School of Hygiene and Tropical Medicine
University of Cambridge
Roche diagnostics