Identifying thresholds for classifying moderate-to-heavy soil-transmitted helminth intensity infections for FECPAKG2, McMaster, Mini-FLOTAC and qPCR

Abstract

The World Health Organization (WHO) has defined moderate-to-heavy intensity (M&HI) infections with soil-transmitted helminths (Ascaris lumbricoides, Trichuris trichiura and the two hookworms, Ancylostoma duodenale and Necator americanus) based on specific values of eggs per gram of stool, as measured by the Kato-Katz method. There are a variety of novel microscopy and DNA-based methods but it remains unclear whether applying current WHO thresholds on to these methods allows for a reliable classification of M&HI infections. We evaluated both WHO and method-specific thresholds for classifying the M&HI infections for novel microscopic (FECPAKG2, McMaster and Mini-FLOTAC) and DNA-based (qPCR) diagnostic methods. For this, we determined method-specific thresholds that best classified M&HI infections (defined by Kato-Katz and WHO thresholds; reference method) in two multi-country drug efficacy studies. Subsequently, we verified whether applying these method-specific thresholds improved the agreement in classifying M&HI infections compared to the reference method. When we applied the WHO thresholds, the new microscopic methods mainly misclassified M&HI as low intensity, and to a lesser extent low intensity infection as M&HI. For FECPAKG2, applying the method-specific thresholds significantly improved the agreement for Ascaris (moderate → substantial), Trichuris and hookworms (fair → moderate). For Mini-FLOTAC, a significantly improved agreement was observed for hookworms only (fair → moderate). For the other STHs, the agreement was almost perfect and remained unchanged. For McMaster, the method-specific thresholds revealed a fair to a substantial agreement but did not significantly improve the agreement. For qPCR, the method-specific thresholds based on genome equivalents per ml of DNA moderately agreed with the reference method for hookworm and Trichuris infections. For Ascaris, there was a substantial agreement. We defined method-specific thresholds that improved the classification of M&HI infections. Validation studies are required before they can be recommended for general use in assessing M&HI infections in programmatic settings.

Fig 1. Agreement in classifying infection intensities for Mini-FLOTAC when applying WHO and method-specific thresholds.

The scatter plots represent the degree of agreement between Mini-FLOTAC and a single Kato-Katz (1x KK; reference method) in classifying infection intensity based on World Health Organization (WHO; Table 1) or the method-specific thresholds (Table 4) for Ascaris, Trichuris and hookworms. The full black dots indicate that intensity of infection was correctly classified as light (bottom left quadrant) and moderate-to-heavy (M&H; top right quadrant). The red dots indicate that intensity of infection was falsely classified as light (bottom right quadrant) and M&H (top left quadrant). The full lines represent the thresholds, the dashed lined represents the line of equality.

Fig 2. Agreement in classifying infection intensities for FECPAK^G2 when applying WHO and method-specific thresholds.

The scatter plots represent the degree of agreement between FECPAK^G2 and a single Kato-Katz (1x KK; reference method) in classifying infection intensity based on World Health Organization (WHO; Table 1) or the method-specific thresholds (Table 4) for Ascaris, Trichuris and hookworms. The full black dots indicate that intensity of infection was correctly classified as light (bottom left quadrant) and moderate-to-heavy (M&H; top right quadrant). The red dots indicate that intensity of infection was falsely classified as light (bottom right quadrant) and M&H (top left quadrant). The full lines represent the thresholds, the dashed line represents the line of equality.

Fig 3. Agreement in classifying infection intensities for McMaster when applying WHO and method-specific thresholds.

The scatter plots represent the degree of agreement between McMaster and a single Kato-Katz (1x KK; reference method) in classifying infection intensity based on either World Health Organization (WHO; Table 1) and method-specific thresholds (Table 4) for Ascaris, Trichuris and hookworms. The full black dots indicate that intensity of infection was correctly classified as light (bottom left quadrant) and moderate-to-heavy (M&H; top right quadrant). The red dots indicate that intensity of infection was falsely classified as light (bottom right quadrant) and M&H (top left quadrant). The full lines represent the thresholds, the dashed line represents the line of equality.

Fig 4. Agreement in classifying infection intensities between Kato-Katz and qPCR.

The scatter plots represent the de degree agreement between qPCR and a single Kato-Katz (1x KK; reference method) in classifying infection intensity cases based World Health Organization (WHO; Table 1) and method-specific thresholds (Table 4) for Ascaris, Trichuris and hookworms. The full black dots indicate that intensity of infection was correctly classified as light (bottom left quadrant) and moderate-to-heavy (M&H; top right quadrant). The red dots indicate that intensity of infection was falsely classified as light (bottom right quadrant) and M&H (top left quadrant). The full lines represent the thresholds, the dashed line represents the line of equality.