The accurate identification of asymptomatic and symptomatic STIs, as well as improvements in the sensitivity and specificity of the syndromic approach, all depend on the availability of diagnostic tests and a screening strategy. High-quality diagnostic tests for STIs are available but are often expensive, frequently labour intensive and, at this stage, not suitable for use as rapid point-of-care tests. This situation is further complicated by a lack of interest from pharmaceutical companies in developing low-cost, high-quality diagnostic tests for diseases that are more prevalent in low- and middle-income countries, since they perceive that the market is not sufficient to recover research and development costs and make a profit.
Gold-standard tests with high levels of sensitivity and specificity are generally used to develop management flow charts and to subsequently evaluate and improve them, but these tests are typically not available for the day-to-day management of people with STIs.
For purposes of equity, diagnostic tests should be affordable, sensitive, specific, user friendly, rapid and robust, equipment-free and deliverable – thus making them accessible. Equally, the tests should be acceptable – which is usually achieved by making the testing procedure minimally invasive.
Currently, however, there are often trade-offs. Nucleic acid amplification testing (NAAT) is very sensitive and specific and can usually be done on non-invasive samples such as urine but often takes 3–4 hours to complete, is expensive and is usually based in a laboratory rather than at a health facility where people present with STI symptoms. Other rapid immunochromatographic strip tests may be less sensitive, but results may be available in 30 minutes or less (38). Two rapid immunochromatographic strip tests for N. gonorrhoeae were evaluated in Brazil and Benin and had sensitivity of 60% and 70%, respectively, specificity of 91% and 97%, respectively, and a positive predictive value of 56% in Brazil (prevalence of N. gonorrhoeae 15%) and 55% in Benin, where the prevalence of N. gonorrhoeae was 5% (39,40). However, it has been suggested that these rapid tests, despite their lower sensitivity, may be as efficient as or more efficient than the more sensitive gold-standard tests in treating gonococcal infections, given the proportion of people who do not return for the results of the more sensitive test. Thus, in day-to-day practice, if longer waiting times or having to return for test results leads to loss to follow-up, the less sensitive tests may well result in more people with STIs receiving treatment (41). Several platforms and assays are being developed to be more portable and easier to operate and to be used at the primary point of care and give rapid turnaround times for results, with accuracy similar to that of laboratory-based NAAT (42–44). Such considerations were deliberated in the STI Guideline Development Group meeting to propose acceptable performance characteristics of prospective point-of-care tests that may improve the treatment flow charts and minimize the undesirable consequences of untreated STIs.
There are several rapid diagnostic tests for screening for syphilis. Most use whole blood, plasma or serum and can be performed within 5–30 minutes. Based on a meta-analysis, sensitivity ranges from 75% to 99% and specificity from 92% to 99% compared with Treponema pallidum haemagglutination assay (TPHA) and Treponema pallidum particle agglutination (TPPA) tests (45). A combination of rapid diagnostic tests for HIV and syphilis (dual HIV and syphilis test) provides potential for increasing syphilis testing. These tests have high sensitivity and specificity, are more cost-effective than a single rapid diagnostic test and are acceptable in terms of turnaround time, cost and a single finger prick (46).
Additionally, already in existence are traditional rapid tests, including microscopy (Gram stain, wet mount and dark field), pH strip tests, the Whiff test using potassium hydroxide solution and the rapid plasma reagin (RPR) test for syphilis. The role of these tests was discussed intensively at the STI Guideline Development Group meeting and assessed in the modelling exercise to explore their utility.
Some of these traditional rapid tests are briefly discussed below.
4.1. Role of microscopy in diagnosing STIs and other reproductive tract infections
The light microscope is used in studying microorganisms, especially for identification purposes. The light microscope uses visible light to directly illuminate specimens, which appear dark against a bright background. In most light microscopes, the image is viewed directly through binocular eyepieces with a magnifying glass and the objective lens located in the revolving nosepiece to give a total magnification ranging from ×10 to ×1000.
The light microscope has been used with Gram staining to detect intracellular gram-negative diplococci within polymorphonuclear leukocytes for the presumptive diagnosis of gonorrhoea. The light microscope is also used to diagnose bacterial vaginosis when Gram-stained vaginal smears show an abundance of gram-positive and gram-negative cocci with reduced gram-positive lactobacilli in the vaginal flora. Another area the light microscope has a role is in wet-mount examination of samples collected from the posterior fornix of the vagina when motile trichomonads can establish a diagnosis of trichomoniasis.
A fluorescence microscope is a type of light microscope that works on the principle of fluorescence. Fluorescence can be used to visualize some bacteria and viruses that are not easily visible by light microscopy following staining by a specific antibody attached to a fluorochrome. In STI detection, the immunofluorescence can be used to detect T. pallidum, C. trachomatis and HSV.
Dark-field microscopy uses a special type of light microscope in which the light beam is split such that the edges of objects in the samples are illuminated so that they appear as silhouettes against a dark background, thus enabling the sample to be seen without stains. In STIs, dark-field microscopes have been used for detecting T. pallidum. However, this has to be performed by well-trained and experienced personnel who can adjust the microscope correctly and can differentiate T. pallidum from other non-pathogenic treponemes and spiral organisms commonly present on genital and anal mucous membranes. Further, since spirochaetes other than treponemes colonize the oral cavity, dark-field microscopy is not recommended for samples from the mouth.
The microscope should be available in any laboratory licensed to perform moderately complex tasks, but it is not usually available as a point-of-care test.
Microscopy for STIs provides a simple, rapid and relatively inexpensive test that can be used near the patient – for example, it can be placed in a procedure room within a primary health care facility. The skills needed for preparing smears for microscopic examination and interpretation of the microscopic image require training and a good working knowledge of the microscope. Further, the microscope should be serviced regularly and should be kept clean and covered when not in use.
4.2. Quality-assured laboratory testing with a fully operational management system
Diagnosing a person with an STI has are serious health implications at the individual and public health levels, and the best available diagnostic tests should therefore be used. All laboratory tests performed and the reports produced for patient management must be of high quality. WHO has developed an implementation tool to assist laboratories in implementing a quality management system (47). To maintain a high-quality service, laboratories should be accredited to a suitable national or international body, such as the International Organization of Standardization. The goal is to achieve compliance with international standard ISO 15189. Such accreditation involves an external audit of the ability to provide a service of high quality by declaring a defined standard of practice, which is confirmed by peer review.
Although this does not include assessment of the appropriateness of the molecular tests chosen for diagnosing STIs, the laboratory is expected to be able to provide evidence of the assessment of the performance capabilities of the tests before they are incorporated into the STI services offered.
Such a laboratory will be considered to have quality-assured molecular testing with a fully operational management system.
In addition, for STI prevention and control, WHO recommends that laboratories aim to offer tests that minimize the time between the sample is taken and the patient receives the results – ideally on the same day as the visit. It is further suggested that laboratories set indicators that reflect the quality of their results with targets for rapid turnaround times.
