Summary of characterized patient admission serum samples and RDT performance in this studya

IFA reciprocal titer cutoff (no. of samples/100 with IFA IgM titer cutoff) and RDT versionNo. of RDT resultsb% Sensitivity (95% CI)% Specificity (95% CI)
≥400 (27)
    PAb142137152 (32–71)97 (90–100)
    MAb143137052 (32–71)96 (88–99)
≥1,600 (17)
    PAb14238182 (57–96)98 (92–100)
    MAb14338082 (57–96)96 (90–99)
≥6,400 (13)
    PAb12418392 (64–100)95 (89–99)
    MAb12518292 (64–100)94 (87–98)
≥25,600 (11)
    PAb115084100 (72–100)94 (87–98)
    MAb116083100 (72–100)93 (86–97)
  • a The results shown are stratified by IFA IgM antibody positivity titers (horizontal rows) with corresponding InBios RDT diagnostic accuracies reported separately for IgM detection modalities, PAb versus MAb. Although the number of characterized samples with confirmed scrub typhus was 21/100, more samples had low IFA IgM positivity, i.e., of all of the samples, if an IFA cutoff titer of ≥1:400 was chosen, then 14 of these were true positives and 13 were false negatives—with rising IFA IgM titers, the rate of false negativity decreased.

  • b RDT results: TP, true positive; FP, false positive; FN, false negative; TN, true negative.