Diagnostic accuracy of Kato-Katz, FLOTAC, Baermann, and PCR methods for the detection of light-intensity hookworm and Strongyloides stercoralis infections in Tanzania

Abstract

Sensitive diagnostic tools are crucial for an accurate assessment of helminth infections in low-endemicity areas. We examined stool samples from Tanzanian individuals and compared the diagnostic accuracy of a real-time polymerase chain reaction (PCR) with the FLOTAC technique and the Kato-Katz method for hookworm and the Baermann method for Strongyloides stercoralis detection. Only FLOTAC had a higher sensitivity than the Kato-Katz method for hookworm diagnosis; the sensitivities of PCR and the Kato-Katz method were equal. PCR had a very low sensitivity for S. stercoralis detection. The cycle threshold values of the PCR were negatively correlated with the logarithm of hookworm egg and S. stercoralis larvae counts. The median larvae count was significantly lower in PCR false negatives than true positives. All methods failed to detect very low-intensity infections. New diagnostic approaches are needed for monitoring of progressing helminth control programs, confirmation of elimination, or surveillance of disease recrudescence.

Figure 1.

Flowchart indicating the number of study participants invited to participate in a helminth screening for the IDEA project in the United Republic of Tanzania between June of 2011 and November of 2012 and the number of stool samples examined with the Kato–Katz thick smear, FLOTAC, Baermann, and PCR methods or a combination thereof for the diagnosis of helminth infections.

Figure 2.

Differences in median hookworm-positive EPG values, median S. stercoralis larvae positive counts, and median positive Ct values in groups of samples identified as true positive or false negative with any other diagnostic method in a study conducted in the United Republic of Tanzania between June of 2011 and November of 2012. *Significant difference (P ≤ 0.05) in the median determined by the Wilcoxon rank sum (Mann–Whitney) test. (A) Difference between hookworm median EPG in true-positive (N = 91) and false-negative (N = 6) FLOTAC samples identified as positive with Kato–Katz (P < 0.001). (B) Difference between hookworm median EPG in true-positive (N = 91) and false-negative (N = 21) Kato–Katz samples identified as positive with FLOTAC (P < 0.001). (C) Difference between hookworm median EPG in true-positive (N = 40) and false-negative (N = 15) PCR samples identified as positive with Kato–Katz (P = 0.438). (D) Difference between hookworm median EPG in true-positive (N = 43) and false-negative (N = 17) PCR samples identified as positive with FLOTAC (P = 0.623). (E) Difference between S. stercoralis median larvae in true-positive (N = 8) and false-negative (N = 38) PCR samples identified as positive with the Baermann method (P = 0.023). (F) Difference between hookworm median Ct values in true-positive (N = 40) and false-negative (N = 15) Kato–Katz samples identified as positive with PCR (P = 0.082). (G) Difference between hookworm median Ct values in true-positive (N = 43) and false-negative (N = 10) FLOTAC samples identified as positive with PCR (P = 0.056). (H) Difference between S. stercoralis median Ct values in true-positive (N = 8) and false-negative (N = 9) Baermann samples identified as positive with PCR (P = 0.194).

Figure 3.

Correlation between hookworm EPG measured with FLOTAC or duplicate Kato–Katz thick smears and Ct values of hookworm real-time PCR in a study conducted in the United Republic of Tanzania between June of 2011 and November of 2012. (A) Correlation between hookworm EPG values measured with FLOTAC and Ct values of hookworm real-time PCR for the detection of N. americanus in fecal samples (N = 211) from coastal Tanzania (Pearson correlation, ρ = −0.30; P < 0.001). (B) Correlation between hookworm EPG values measured with duplicate Kato–Katz thick smears and PCR Ct values of hookworm real-time PCR for the detection of N. americanus in fecal samples (N = 215) from coastal Tanzania (ρ = −0.36; P < 0.001).