Latest Technological Advancements for STI Detection
Point-of-care and molecular assays can improve early detection of sexually transmitted infections
More than 30 bacterial, viral, and parasitic pathogens are transmissible through sexual contact and constitute a group of infections referred to as sexually transmitted infections (STIs). If untreated, these infections can lead to serious health consequences, including chronic pelvic pain, infertility, and adverse pregnancy outcomes. In addition, STIs significantly impact the economy and health care system.
Given the high prevalence of asymptomatic infections and the limitations of diagnosis based only on clinical presentation, laboratory testing is essential to the early and effective treatment of STIs. The appropriateness of a diagnostic tool relies not only on the tool's accuracy, but also on the technical requirements for testing, cost, throughput, and testing purpose.
This article outlines the latest technological advancements for detecting STIs, highlighting the advantages, shortcomings, and point-of-care (POC) potential of available methods and technologies.
Detecting STIs via microscopy
Direct microscopy to demonstrate the presence of an organism is still used in resource-poor settings and in peripheral or intermediate-level laboratories designed to provide a rapid on-site diagnosis of STIs.
When performed while patients are present, microscopy may provide immediate results to guide decisions around patient care and treatment. However, like other tests, it requires specialized equipment (a microscope) and may require electrical power or special stain procedures.
In addition, the test’s sensitivity is affected by the microscopist's skills and tends to decrease in asymptomatic patients, as seen in gonorrhea and trichomoniasis. Moreover, the microscopic analysis should be carried out within 10 minutes of sample collection, making the examination unsuitable for high-throughput laboratories.
Detecting STIs via culturing organisms
After isolation from genital fluids, laboratories can culture and identify the organisms that cause gonorrhea, bacterial vaginosis, candidiasis, chancroid, chlamydiosis, herpes, and genital mycoplasmas.
The pathogens that cause gonorrhea, bacterial vaginosis, and candidiasis are easily cultured.
In contrast, those that cause chancroid, chlamydiosis, herpes, and genital mycoplasmas are more difficult to grow and are instead frequently identified immunologically or by molecular biology.
The microorganisms that cause syphilis, human immunodeficiency virus (HIV), and trichomoniasis are usually not cultured because they do not grow on artificial culture mediums.
Despite being a slow process, culturing organisms is sensitive and highly specific in optimized circumstances. In addition, culture remains the most traditional method to test for antimicrobial resistance.
MALDI-TOF MS: Improving the efficiency and precision of culturing
The use of MALDI-TOF mass spectrometry (MS) has improved the efficiency and precision of culture. This approach can precisely identify fungi and bacteria directly from colonies grown in culture within 6-10 minutes and for low costs.
Culturomics, a high-throughput culture method that uses MALDI-TOF MS to identify vaginal microbiota, also has potential applications to STIs. The culturomic approach has provided exciting new perspectives on how microorganisms interact with each other, the host, and pathogens. For example, a detailed characterization of microbiota-associated factors can help to assess the impact of single-bacterial isolates from the vaginal community on vaginal health and the containment of STIs.
Detecting STIs via immunological assays
Immunoassays detect either the pathogen’s antigens or the presence of antibodies produced by a patient’s immune system in response to infection. These tools can be adapted to POC formats, like the OSOM® Trichomonas Rapid Test for trichomoniasis.
Serological assays for the presence of antibodies also represent a sensitive and specific tool for diagnosing syphilis. These tests, mostly immunochromatographic strip-based assays, can now detect both treponemal and nontreponemal antibodies, allowing discrimination between past infection and active syphilis.
Immunoassays are unavailable for other STIs such as genital mycoplasmas and HPV (human papillomavirus). In addition, they may exhibit low sensitivity for detecting infections due to Chlamydia trachomatis and Neisseria gonorrhoeae, and should be used only when other detection methods are unavailable.
Multiplexed immunoassays for STI detection
The development of multiplexed immunoassays is an emerging trend. It might be helpful to provide timely access and fill screening gaps for simultaneous testing in outpatient clinics or mobile units at or near the point of care.
For example, Soler and colleagues illustrated the development of a nanoplasmonic biosensor integrated with microfluidics for rapid and simultaneous antibody detection of C. trachomatis and N. gonorrhoeae in urine, without amplification steps.
Simultaneous antibody detection of Treponema pallidum and HIV can also be accomplished through various immunochromatographic POC tests with excellent sensitivity and specificity for both pathogens.
Mobile point-of-care tests for STI detection
A microfluidic-based diagnostic test for HIV and syphilis that attaches to (and is powered by) the iPod’s headphone jack was described by Laksanasopin and colleagues. The test is based on the standard immunoassay but uses gold-labeled antibodies to detect HIV and syphilis antigens in only 2 μL of whole blood, and then silver reagents to amplify the resulting signal. The mobile test produces results in less than 15 minutes and has the sensitivity and specificity needed to make treatment decisions in the field.
The development of automated strip readers and the use of artificial intelligence also holds promise to reduce mistakes in visual interpretation of immunochromatographic assay.
Detecting STIs with nucleic acid amplification tests
Methods based on amplifying and detecting genetic material (DNA or RNA) of pathogenic organisms now constitute the gold standard for identifying many STIs. Nucleic acid amplification tests (NAATs) are crucial for detecting pathogens not identified or too expensive to detect using traditional tools, such as C. trachomatis, genital mycoplasmas, and some viruses.
This approach includes PCR-based and isothermal amplification assays. Moreover, novel methods highlight the possibility of sensitive detection without amplification, which reduces sample handling and processing times and makes POC testing possible.
What are the advantages of using NAATs for STI detection?
NAATs overcome some critical limitations of microscopy and culture-based approaches. NAATs can be easily automated, facilitating high-throughput testing, have increased specificity and sensitivity compared to other methods, and often detect pathogens even when the material of interest is present in small amounts, —an advantage for detecting infections in specimens with low viral or bacterial loads, such as in asymptomatic patients.
Due to their high sensitivity, NAATs can also help detect extragenital infections, as shown in the case of rectal or pharyngeal gonorrhea in men. Furthermore, NAAT technology can use specimens (such as urine or vaginal swabs) provided by clients, eliminating the need for an examination from a health care provider.
What are the drawbacks of NAATs?
Although NAATs have improved the diagnosis of STIs, the high costs and laboratory infrastructure required limit accessibility, particularly in low- and middle-income countries. However, cheaper and faster NAAT-based tools are being developed, providing an opportunity to design POC NAAT systems with an equivalent diagnostic accuracy of laboratory-based tests.
One of these assays is the POC Xpert CT/NG performed on the GeneXpert System (Cepheid, Sunnydale, California, US), a qualitative, in vitro real-time PCR test for rapid detection (results in less than two hours) of DNA from C. trachomatis and N. gonorrhoeae. The assay and platform have been CE marked and cleared by the U.S. FDA for use in urine, endocervical, vaginal, rectal, and pharyngeal specimens.
Other promising POC diagnostic tests are currently under development.
For example, the mobiNAAT, is a mobile phone–based NAAT that uses a small magnetofluidic cartridge to perform isothermal amplification. The platform delivers results in about one hour. A prototype of the device testing for C. trachomatis was evaluated in an emergency room setting and the results showed 100 percent concordance with the standard-of-care NAAT.
Detecting STIs via next-generation DNA sequencing
Through the use of next-generation sequencing (NGS), the emerging field of clinical metagenomics has the potential to revolutionize pathogen detection. This technique allows the simultaneous identification of all microorganisms in a same sample, without a priori knowledge of their identities.
This culture-independent method constitutes a rapid approach that provides comprehensive medically-actionable information, including the presence or absence of microorganisms, their species, and the detection of antimicrobial resistance determinants, guiding antibiotic therapy and positively impacting patient management.
In the STIs space, clinical metagenomics currently allows for the identification of specific bacterial species, such as Gardnerella vaginalis and Atopobium vaginae, which are predictive for bacterial vaginosis. Although bacterial vaginosis is not an STI, it can increase the risk of contracting STIs and of adverse pregnancy outcomes.
The pros and cons of NGS-based STI detection
Clinical metagenomics is more expensive than traditional culture or NAATs. However, that could be balanced against i) the cost savings of not needing to order other tests because NGS facilitates universal pathogen detection, ii) being able to use targeted therapies much earlier, and iii) the benefit of earlier patient discharge. As such, clinical metagenomics may one day replace conventional methods and be a game-changing technology for POC applications in clinical medicine and public health.
Nonetheless, many challenges remain before the wide adoption of clinical metagenomics. Among others, these include developing appropriate laboratory workflows and standards to avoid sample contamination, reducing the cost and turnaround time of the instrumentation, and establishing more user-friendly bioinformatics tools.
Improving STI diagnosis and reducing treatment delays
We have witnessed rapid developments in the laboratory detection of STIs. These molecular techniques, some of which are available at the POC, have provided physicians with an array of diagnostic options.
Although microscopy, culture, and immunoassays continue to have a critical role for detecting STIs, because NAAT-based systems have revolutionized our ability to detect pathogens in clinical samples, they now represent the industry gold standard in STI testing.
In recent years, clinical metagenomics has emerged as a promising diagnostic method in microbiology and could also be a valuable tool for identifying and treating STIs.
Altogether, these technological advancements can improve the diagnosis of STIs and reduce treatment delays, leading to public health benefits by interrupting disease transmission.