Screening performance of influenza definitions for detecting seropositive status (sensitivity, specificity, PPV and NPV)
| Measure | NZ sentinel surveillance ILI definition* | NZ Ministry of Health ILI definition† | Self-reported flu status‡ |
| Point estimate (95% CI) | Point estimate (95% CI) | Point estimate (95% CI) | |
| Adults (18+ years)§ (n=413) | |||
| Sensitivity | 37.7 (25.5 to 50.0) | 38.0 (25.6 to 50.4) | 45.7 (33.0 to 58.3) |
| Specificity | 60.5 (53.6 to 67.4) | 67.2 (60.6 to 73.8) | 58.1 (51.0 to 65.3) |
| PPV | 21.6 (13.6 to 29.6) | 25.1 (15.8 to 34.4) | 24.1 (16.2 to 31.9) |
| NPV | 77.1 (70.6 to 83.5) | 79.0 (73.1 to 84.8) | 78.7 (72.0 to 85.3) |
| Children (<18 years)¶ (n=318) | |||
| Sensitivity | 32.5 (21.0 to 44.0) | 36.0 (24.5 to 47.6) | 42.4 (30.0 to 54.9) |
| Specificity | 68.4 (58.4 to 78.4) | 57.2 (47.3 to 67.1) | 52.6 (42.1 to 63.0) |
| PPV | 43.4 (29.1 to 57.7) | 38.6 (27.0 to 50.1) | 40.9 (29.3 to 52.6) |
| NPV | 57.6 (48.2 to 67.0) | 54.5 (44.0 to 65.0) | 54.1 (43.0 to 65.3) |
| Healthcare workers** (n=423) | |||
| Sensitivity | 32.5 (23.8 to 41.1) | 30.7 (22.2 to 39.2) | 48.2 (38.8 to 57.6) |
| Specificity | 70.8 (65.7 to 75.9) | 76.4 (71.6 to 81.2) | 57.7 (51.9 to 63.4) |
| PPV | 29.9 (21.9 to 37.9) | 32.7 (23.8 to 41.6) | 30.5 (23.6 to 37.3) |
| NPV | 73.5 (68.5 to 78.6) | 74.7 (69.8 to 79.5) | 74.3 (68.6 to 80.1) |
↵* Two or more symptoms from: fever, muscle ache and headache.
↵† Fever, plus cough and/or sore throat.
↵‡ Self-diagnosis, assisted-self-diagnosis or self-diagnosis by proxy.
↵§ Adults: 16 ‘don't know’ respondents on self-reported influenza status were excluded from analysis.
↵¶ Children: 21 ‘don't know’ respondents on self-reported influenza status were excluded from analysis.
↵** Healthcare workers: 22 ‘don't know’ respondents on self-reported influenza status and five respondents missing immunological status were excluded from analysis.
ILI, influenza like illness; NPV, negative predictive value; NZ, New Zealand; PPV, positive predictive value.