Table 3. Summary of diagnostic test results in the 50 patients screened positive by the second blood-IgM ELISA.
| Days from fever onset to the 1st blood collection (median, IQR) | 8, 7–13 |
| Days from fever onset to the 2nd blood collection (median, IQR) | 14, 11–20 |
| Days of interval between the 1st and 2nd blood collection (median, IQR) | 5, 4–6 |
| Number of patients with ST diagnosis | 26 |
| IgM or IgG titer ≥ 4-fold increase by IIP (with positive by any PCR) | 14 (4) |
| Single IgM titer ≥ 320 (i.e., ≥ 1:320) by IIP (with positive by any PCR) | 12 (8) |
| Positive by more than one PCR assays* (with single IgM titer ≥ 320 by IIP) | 10 (6) |
| Dominant IIP reactions against a single antigen | 15 |
| against Karp antigen | 8 |
| against Kato antigen | 0 |
| against Gilliam antigen | 4 |
| against Hirano/Kuroki antigen | 1 |
| against Irie/Kawasaki antigen | 1 |
| against Shimokoshi antigen | 1 |
| Indistinguishable IIP reactions against multiple antigens | 11 |
| IgM ELISA OD value of the 1st blood sample (median, IQR) | 0.534, 0.384–1.748 |
| IgM ELISA OD value of the 2nd blood sample (median, IQR) | 1.070, 0.623–3.217 |
| AUC of ROC curve of ST diagnosis by IgM ELISA in the 1st blood samples (95% CI) | 0.73 (0.57–0.89) |
| Diagnostic parameters by cut-off OD value at 0.513 in the 1st blood sample-IgM ELISA | |
| accuracy (95% CI) | 68% (53.3%-80.5%) |
| sensitivity (95% CI) | 69.2% (48.2%-85.7%) |
| specificity (95% CI) | 66.7% (44.7%-84.4%) |
| positive predictive value (95% CI) | 69.2% (48.2%-85.7%) |
| negative predictive value (95% CI) | 66.7% (44.7%-84.4%) |
| Diagnostic parameters by cut-off OD value at 1.576 in the 1st blood sample-IgM ELISA# | |
| accuracy (95% CI) | 78% (64.0%-88.5%) |
| sensitivity (95% CI) | 57.7% (36.9%-76.6%) |
| specificity (95% CI) | 100% (85.8%-100%) |
| positive predictive value (95% CI) | 100% (78.2%-100%) |
| negative predictive value (95% CI) | 68.6% (50.7%-83.1%) |
| AUC of ROC curve of ST diagnosis by IgM ELISA in the 2nd blood samples (95% CI) | 0.91 (0.83–0.99) |
| Diagnostic parameters by cut-off OD value at 1.599 in the 2nd blood sample-IgM ELISA# | |
| accuracy (95% CI) | 84% (70.9%-92.8%) |
| sensitivity (95% CI) | 73.1% (52.2%-88.4%) |
| specificity (95% CI) | 95.8% (78.9%-99.9%) |
| positive predictive value (95% CI) | 95% (75.1%-99.9%) |
| negative predictive value (95% CI) | 76.7% (57.7%-90.1%) |
* All the PCR-positive patients also fulfilled the diagnostic criteria by IIP.
# The presented cut-off OD values at 1.576 and 1.599 produced the highest accuracy in the ROC analyses.
IQR: interquartile range, ST: scrub typhus, IIP: indirect immunoperoxidase assay, OD: optical density, AUC: area under the curve, ROC: receiver operating characteristic, CI: confidence interval