Loop-Mediated Isothermal Amplification (LAMP) for the Diagnosis of Sexually Transmitted Infections: A Review

Abstract

Sexually transmitted infections (STIs) remain a significant public health concern. Given the asymptomatic nature of many STIs, diagnostic testing is critical for determining the appropriate treatment, enabling effective tracing and reducing the risk of further transmission. Nucleic acid amplification tests (NAATs) are the most sensitive and the most widely used in well-resourced settings. The majority of available NAATs are based on polymerase chain reaction (PCR), which requires highly trained personnel and costly equipment, making it impractical for resource-limited settings. Loop-mediated isothermal amplification (LAMP) has emerged as a simple, rapid, sensitive and low-cost alternative for pathogen detection, particularly well-suited for point-of-care tests (POCT). In this review, we evaluate LAMP assays reported in the literature for the detection of pathogens linked to the high incidence STIs prioritised by the World Health Organization (WHO) for POCT in 2023. These include Neisseria gonorrhoeae, Chlamydia trachomatis, Trichomonas vaginalis, T. pallidum subspecies pallidum, as well as other common STIs such as herpes simplex virus, hepatitis B virus and human immunodeficiency virus (HIV). For each LAMP assay, we identified and summarised the key elements such as the type and number of tested clinical specimens, chosen target gene, detection system, reference test and clinical outcomes. We highlight the advantages and limitations of these assays and discuss the gaps that should be addressed to improve their applicability for POCT.

Keywords: Chlamydia trachomatis; Neisseria gonorrhoeae; Trichomonas vaginalis; T. pallidum subspecies pallidum; hepatitis B virus; herpes simplex virus; human immunodeficiency virus (HIV); loop‐mediated isothermal amplification (LAMP); point‐of‐care tests (POCT); sexually transmitted infections (STIs).

FIGURE 1

Schematic representation of the mechanism of LAMP (left) and common detection methods (right). The left figure shows the process that starts from primer FIP, however, DNA synthesis can also begin from primer BIP. The formation of a dumbbell‐shaped structure (iv) acts as a seed for exponential amplification for generation of amplicons of various sizes. As shown in the right figure, different detection methods can be applied for results readout as indicated by amplicon and by‐products. For amplicon detection, the most common method is to include intercalating fluorescent dyes such as SYBER Green or Syto9 in the LAMP reaction; amplification can be monitored by real‐time/end‐point fluorescence or alternatively detected visually under UV or blue light. Visual fluorescence detection under visible light is also possible, however, the colour change (yellow/orange to light green) is subtle. With labelled primers, amplicons can be also detected on a lateral flow detection (LFD) strip. Furthermore, some detection methods target by‐products namely pyrophosphate and hydrogen ions. Pyrophosphate precipitates with certain metal ions such as Mg²⁺, Zn²⁺ and Mn²⁺; the metal ion depletion can be further detected using colorimetric metal indicators such as hydroxy naphthol blue (HNB) (Goto et al. 2009) and pyridylazophenol dyes (PAPS) (Zhang, Hunt, et al. ; Szobi et al. 2023) or alternatively by using fluorescent metal ion indicators such as calcein (Tomita et al. 2008). Additionally, the precipitation of magnesium pyrophosphate results in a change in the turbidity of the solution, which can be detected by a turbidimeter (Mori et al. 2001). For hydrogen ion detection, a colorimetric pH indicator such as phenol red (Tanner et al. 2015) can be incorporated in the reaction. In addition, the release of hydrogen ions can be detected by electrochemical measurements (Wang et al. 2024).

FIGURE 2

Flow diagram detailing the selection and review process. CT, Chlamydia trachomatis ; HBV, hepatitis B virus; HIV, human immunodeficiency virus; HSV, herpes simplex virus; NG, Neisseria gonorrhoeae ; Syphilis, T. pallidum subspecies pallidum; TV, Trichomonas vaginalis.