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Point of care tests for resource-limited settings

The frequent occurrence of asymptomatic infection means that chlamydial infections cannot be reliably excluded without the use of sensitive and accurate diagnostic tests. The best diagnostic tests based on nucleic acid amplification have achieved excellent positive and negative predictive values, even in low prevalence settings. However they are laboratory tests, distant from the clinic or home-based patient setting. They require skilled laboratory personnel and results are not available while the patient waits or to inform initial treatment. Over the years a number of relatively rapid point of care tests have been developed. In the context of rapidity, indications are that patients will wait up to an hour for a result, but not significantly longer. Rapid tests include the Unipath Clearview^® , the Kodak Surecell^® the Quidel QuickVue^® the Biostar OIA^® and the Handilab^® Chlamydia C.  There have been surprisingly few studies of rapid chlamydia tests, perhaps because the general impression is that these qualitative tests have considerably lower sensitivity than the best laboratory-based nucleic acid amplification tests [Pate et al., 1998; Rani et al., 2002; Yin et al., 2007].   However as Lee (2005) has pointed out, in a population with a reasonable prevalence of infection a small loss of sensitivity might be an acceptable trade-off against the advantages of rapid clinic based testing and effectively targeted treatment. Helen Lee has a commercial diagnostics background, having worked with Abbott Diagnostics on the development of their chlamydial diagnostics. Subsequently she moved to Cambridge University to develop rapid point of care diagnostics for resource-poor settings, funded in part by the Wellcome Trust. This has resulted in the development of a university spin-off company, Diagnostics for the Real World Ltd (and a US counterpart at Sunnyvale California) whose first product is the Chlamydia Rapid Test. From the sparse details which I have been able to glean from public sources, this is based on a second generation signal amplification enzyme immunoassay for chlamydial lipopolysaccharide in a dipstick-type format. In this respect it is not dissimilar to the Unipath Clearview^®. Results are available in about 30 minutes. It seems likely it is manufactured, at least in part, in the Far East. Initial results indicate considerable promise for this assay in populations with a reasonable prevalence of infection.

Michel et al., 2006 in a study of active trachoma found that the Chlamydia Rapid Test was a more valuable indicator of ocular chlamydial infection than the presence of trachomatous inflammation and follicles (WHO grading: TF). Against commercial PCR detection of ocular C. trachomatis infection as gold standard,  TF had a sensitivity of 64.1% and specificity of 80.2% and positive predictive value of 43.6%. By contrast, the Chlamydia Rapid Test had a sensitivity of 83.6%, a specificity of 99.4% (98.8-100.0), and positive predictive value of 97.3%. It was concluded that the Chlamydia Rapid Test was more valuable than TF clinical signs as the basis for targeted antibiotic therapy for the prevention of active trachoma.

Mahilum-Tapey et al., 2007 studied the performance of this test versus polymerase chain reaction (PCR) or strand displacement assay (SDA) tests in 1349 women attending either a young person's sexual health centre or two genitourinary medicine clinics. PCR positivity rates for C. trachomatis infection were between 6 to 9%. Compared with PCR, the resolved sensitivity, specificity, positive predictive value, and negative predictive value of the Chlamydia Rapid Test were 83.5% (91/109), 98.9% (1224/1238), 86.7% (91/105), and 98.6% (1224/1242). Compared with SDA, sensitivity and specificity of the Chlamydia Rapid Test were 81.6% (40/49) and 98.3% (578/588). The organism load on self collected vaginal swabs ranged from 5.97x10² to 1.09x10⁹ Chlamydia plasmids per swab, which correlated well with the Chlamydia Rapid Test's visual signal (r=0.6435, P<0.0001). The authors concluded that the performance of the Chlamydia Rapid Test with self collected vaginal swabs indicated that it would be an effective same day diagnostic and screening tool for C. trachomatis infection in women.

In contrast Yin et al., 2007 studied 1497 women at six genitourinary medicine clinics in urban China with a prevalence of C. trachomatis genital tract infection of 13.2% (by PCR). The Clearview Chlamydia MF rapid test had a sensitivity of 32.8% and 49.7% for vaginal and cervical specimens versus PCR and corresponding specificities of 99.2% and 97.9%. It was concluded that the test, while rapid and convenient, had unacceptably low sensitivity. The Handilab C rapid test also had unacceptable low sensitivity when evaluated in Norway [Moi, 2007].

There is increased interest in the development of rapid, point of care tests as demonstrated by the WHO TDR STD Diagnostics Initiative who have made recommendations for the evaluation of such tests [Herring et al., 2006]. Initial reports of the Chlamydia Rapid Test indicate it is substantially more sensitive than previous rapid tests. However it is still early days for this test. Other issues such as shelf life, cost and manufacturing consistency are also important. Further evaluations of this test are urgently needed.

[MEW] February 2008

NEXT: Classic diagnostics: Serology

References

Lee, H. (2005). Developing diagnostics for resource-limited settings. On-line presentation in pdf format covering the Chlamydia Rapid Test among others

Mahilum-Tapey, L., Laitila, V., Wawrzyniak, J. J., Alexander, S., Ison, C., Swain, A., Barber, P., Ushiro-Lumb, I. & Gih, B. T. (2007). New point of care Chlamydia Rapid Test--bridging the gap between diagnosis and treatment: performance evaluation study. British Medical Journal 335, 1190 - 1194. Epub 2007 Nov 30. Full paper

Michel, C. E. et al., (2006). Field evaluation of a rapid point-of-care assay for targeting antibiotic treatment for trachoma control: a comparative study. Lancet 367(9522):1585 - 1590

Moi, H. (2007). Handilab C Chlamydia for home testing is not what it claims. Tidsskr Nor Laegeforen 127, 2083 - 2085 (Norwegian)

Pate, M. S., Dixon, P. B., Hardy, K., Crosby, M. & Hook, E. W. (1998). Evaluation of the Biostar Chlamydia OIA assay with specimens from women attending a sexually transmitted disease clinic. Journal of Clinical Microbiology 36, 2183 - 2186. Full paper

Rani, R., Corbitt, G., Killough, R., Curless, E. (2002). Is there any role for rapid tests for Chlamydia trachomatis? International Journal of STD and AIDS 13, 22 - 24. [Interesting pilot study].

Yin, Y. P. et al., (2006). Clinic-based evaluation of Clearview Chlamydia MF for detection of Chlamydia trachomatis in vaginal and cervical specimens from women at high risk in China. Sexually Transmitted Infections 82, Supplement 5, 33 - 37. Epub 2006 Nov 22.  

NEXT: Classic diagnostics: Serology