Nucleic Acid Amplification Tests in the Diagnosis of Sexually Transmitted Diseases
Based on 81 scientific papers published between 2010 and 2021, experts from Johns Hopkins University in Baltimore have prepared an overview of the current epidemiology, diagnosis, and treatment of sexually transmitted diseases (STD). We summarize information on the diagnosis of gonorrhea, chlamydial infections, M. genitalium, trichomoniasis, genital infections by HSV-1/2, and HPV using nucleic acid amplification tests (NAAT), the most well-known method being PCR. NAAT shows high sensitivity and specificity in testing for these diseases.
Gonorrhea
Gonorrhea, also known as the clap, is caused by the gram-negative diplococcus Neisseria gonorrhoeae. Infection by this obligate pathogen most commonly manifests as a purulent inflammation of the urogenital tract, rectum, nasopharynx, or conjunctiva (in the case of newborn infection during birth).
For the detection of gonorrhea in both genital and extragenital sites, NAAT, which differ in amplification methods and target nucleic acid, have a sensitivity > 90% and a specificity of at least 98%. For women, the preferred sample is a vaginal swab (including self-swab), with cervical swabs and first-catch urine (first 20–30 ml) also acceptable. For extragenital sites, NAAT is recommended for specimens from the pharynx and rectum. In men who have sex with men, gonococcal infection is detected in 14–85% of cases only at extragenital sites, and in women, 6–50%.
In contrast, culture has a sensitivity of 50–85%, is suitable only for samples from the endocervix and urethra, and requires special transport conditions. However, this technique is necessary to determine antimicrobial susceptibility in cases of suspected resistance. Gram staining is suitable only for urethral samples from symptomatic men, where it shows a specificity of 99% and sensitivity > 95% for detecting urethral infections. Sensitivity in samples from women and asymptomatic men is insufficient.
Chlamydia
Gram-negative anaerobic bacteria Chlamydia trachomatis replicate in eukaryotic cells. The genus C. trachomatis can be categorized into 18 serotypes. Serotypes A, B, Ba, and C cause trachoma—a chronic keratoconjunctivitis that can lead to blindness, endemic in Asia and Africa. Sexually transmitted urogenital chlamydial infection is caused by serotypes D–K; infection with serotypes L1–L3 leads to lymphogranuloma venereum accompanied by typical genital ulcers.
For chlamydial infection diagnosis, NAAT is the test of choice for swabs from all potentially infected sites, and the testing approach is similar to that for gonorrhea. Results are usually positive in cases of lymphogranuloma venereum, but do not differentiate between serotypes L1–L3 and D–K.
Mycoplasma genitalium
It is estimated that the sexually transmitted infection by Mycoplasma genitalium bacteria may cause up to 20% of nongonococcal urethritis, cervicitis, and postpartum pelvic inflammatory disease.
NAAT can be used to test urine, urethral swabs, external urethral meatus swabs in men, and cervix and vaginal swabs. Testing for M. genitalium at extragenital sites and in asymptomatic individuals is not appropriate. Testing is recommended for men with persistent nongonococcal urethritis and women with persistent cervicitis.
Trichomoniasis
This disease is caused by the protozoan Trichomonas vaginalis. The infection is asymptomatic in about three-quarters of cases; in women, it may present as vulvovaginitis or dysuria, while in men, dysuria and urethral discharge are common symptoms.
NAAT are the preferred diagnostic tests with the highest sensitivity for testing vaginal, cervical, and urine samples in women. In men, it can be used for urine, urethral, or external urethral meatus swabs. Trichomonads are commonly diagnosed microscopically in women, but sensitivity using vaginal samples is only 51–65%. For Papanicolaou smears, sensitivity is 61%. Culture is necessary for determining antimicrobial susceptibility. The sensitivity of rapid antigen tests from vaginal swabs is approximately 82–90%.
Herpes simplex virus
Genital herpes is caused by HSV viruses. HSV-2 is primarily an anogenital pathogen, while HSV-1 also causes orolabial infections. Up to 70% of infections are asymptomatic.
When swabs from lesions are used, NAAT has a sensitivity of 96.7–100%. The sensitivity of culture ranges from 30 to 70%, so it is only used if NAAT is unavailable and to test for antiviral sensitivity. In the absence of lesions, serological testing for IgG antibodies against the specific glycoprotein G antigen can be used. These antibodies are typically detectable 2 weeks to 3 months after the initial infection.
Human papillomavirus and HPV screening in the Czech Republic
Another sexually transmitted virus is the human papillomavirus (HPV), a non-enveloped virus with circular double-stranded DNA. The infection may present as cutaneous or anogenital warts, which depending on the virus subtype, can progress to a malignancy. Subtypes 16 and 18 are particularly high-risk as they can cause intraepithelial lesions with a high potential for malignancy.
In the Czech Republic, every woman from the age of 15 is entitled to an annual gynecological preventive examination, including cervical cancer screening via cervical cytological, bacteriological, or virological testing. HPV testing (for the presence of high-risk HPV nucleic acids in a cervical swab) is covered in cases of ambiguous cytological findings and, since 2021, for all women at ages 35 and 45 as a highly sensitive test during negative cytological screening. This globally recommended extension of screening to include an HPV test for specific age groups significantly increases the sensitivity and safety of cervical cancer screening. Sample collection in this age group is performed during standard gynecological preventive examinations concurrently with a smear for cytological screening.
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Sources:
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