Diagnostic Performance of Loop-Mediated Isothermal Amplification and Ultrasensitive Rapid Diagnostic Tests for Malaria Screening Among Pregnant Women in Kenya

Abstract

Background: Screen-and-treat strategies with sensitive diagnostic tests may reduce malaria-associated adverse pregnancy outcomes. We conducted a diagnostic accuracy study to evaluate new point-of-care tests to screen pregnant women for malaria at their first antenatal visit in western Kenya.

Methods: Consecutively women were tested for Plasmodium infection by expert microscopy, conventional rapid diagnostic test (cRDT), ultra sensitive RDT (usRDT), and loop-mediated isothermal amplification (LAMP). Photoinduced electron-transfer polymerase chain reaction (PET-PCR) served as the reference standard. Diagnostic performance was calculated and modelled at low parasite densities.

Results: Between May and September 2018, 172 of 482 screened participants (35.7%) were PET-PCR positive. Relative to PET-PCR, expert microscopy was least sensitive (40.1%; 95% confidence interval [CI], 32.7%-47.9%), followed by cRDT (49.4%; 95% CI, 41.7%-57.1), usRDT (54.7%; 95% CI, 46.9%-62.2%), and LAMP (68.6%; 95% CI, 61.1%-75.5%). Test sensitivities were comparable in febrile women (n = 90). Among afebrile women (n = 392), the geometric-mean parasite density was 29 parasites/µL and LAMP (sensitivity = 61.9%) and usRDT (43.2%) detected 1.74 (95% CI, 1.31-2.30) and 1.21 (95% CI, 88-2.21) more infections than cRDT (35.6%). Per our model, tests performed similarly at densities >200 parasites/µL. At 50 parasites/µL, the sensitivities were 45%, 56%, 62%, and 74% with expert microscopy, cRDT, usRDT, and LAMP, respectively.

Conclusions: This first-generation usRDT provided moderate improvement in detecting low-density infections in afebrile pregnant women compared to cRDTs.

Keywords: diagnostic sensitivity in malaria in pregnancy; loop-mediated isothermal amplicification for malaria‌; malaria in pregnancy; malaria screening at first antenatal care clinic visit; ultrasensitive rapid diagnostic tests for malaria.

Figure 1.

Distribution of PET-PCR positive samples by parasite density stratified by fever status, gravidity and trimester of pregnancy Samples are plotted as the kernel density by log₁₀-transformed parasites/μL according to (A) fever status, (B) gravidity, and (C) trimester of pregnancy. Abbreviation: PET-PCR, photo-induced electron-transfer polymerase-chain-reaction.

Figure 2.

Distribution of positive samples by diagnostic test and modelled sensitivity to PET-PCR at densities below 500 parasites/μL (A) Venn diagram of P. falciparum positivity by PET-PCR, microscopy, RDT, us-RDT, and LAMP. PET-PCR was the reference test. (B) Logistic modelled probability of test sensitivity and 95% credible intervals (shaded area) by log₁₀-transformed parasite density calculated by PET-PCR for each diagnostic. Only samples with calculated densities below 500 parasites/μL are considered in the model. Abbreviations: PET-PCR, photo-induced electron-transfer polymerase-chain-reaction; cRDT, conventional RDT; usRDT, ultra-sensitive RDT; LAMP, loop-mediated isothermal amplification.

Figure 3.

Curves of modelled test sensitivity at low parasite density with PET-PCR as the reference Sensitivities of diagnostic tests at low density derived from logistic models using PET-PCR positive samples with parasite densities below 500 parasites/μL. The vertical axis represents the modelled sensitivity of the test. Models and sensitivity outputs are stratified by (A) fever status, (B) gravidity, and (C) trimester of pregnancy. Abbreviations: 95% CI, 95% confidence interval; parasites/μL, parasites per microliter; PET-PCR, photo-induced electron-transfer polymerase-chain-reaction; cRDT, conventional RDT; usRDT, ultra-sensitive RDT; LAMP, loop-mediated isothermal amplification

Figure 4.

Relative test diagnostic sensitivity to PET-PCR by febrile status and among afebrile women by trimester of pregnancy and gravidity Sensitivities of tests were calculated using PET-PCR as the reference test. Sensitivity ratios were modelled using Poisson regression. RRs greater than 1 indicate that test A is more sensitive than test B for the given criteria. Calculations are stratified by all PET-PCR positives, all febrile women, all afebrile women, and afebrile women in the first, second, or third trimester of pregnancy, respectively.. Abbreviations: TP, true positives within sub-group by PET-PCR and percent of total positive population; ND, number of true positives detected by the given test; Sn (95% CI), sensitivity (95% confidence interval); SR, sensitivity ratio; GMPD, geometric mean parasite density. PET-PCR, photo-induced electron-transfer polymerase-chain-reaction; cRDT, conventional RDT; usRDT, ultra-sensitive RDT; LAMP, loop-mediated isothermal amplification.