Effectiveness of Flag-4 registration and targeted latent tuberculosis (TB) infection screening
| Age groups at primary care registration | ||||||
|---|---|---|---|---|---|---|
| All | <16 | 16–35 | 36+ | |||
| WHO TB incidence region/100 000 population | All | NNS* (95% CI) | 145 (130 to 162) | 465 (294 to 746) | 121 (107 to 137) | 129 (102 to 162) |
| (N (preventable cases)/N (cohort))† | (250/49162) | (15/9063) | (175/30369) | (60/9730) | ||
| % Preventable cases‡ | 29.2 | 1.8 | 20.4 | 7.0 | ||
| <150 | NNS* (95% CI) | 1546 (925 to 2625) | 5291 (814 to 100000) | 1003 (600 to 1704) | No events | |
| (N (preventable cases)/N (cohort))† | (10/24326) | (1/5283) | (8/15729) | |||
| % Preventable cases‡ | 1.2 | 0.1 | 0.9 | |||
| 150–499 | NNS† (95% CI) | 80 (71 to 90) | 345 (176 to 704) | 65 (57 to 74) | 82 (65 to 105) | |
| (N (preventable cases)/N (cohort))† | (226/22 510) | (9/3137) | (159/13 569) | (58/5804) | ||
| % Preventable cases‡ | 26.4 | 1.1 | 18.6 | 6.8 | ||
| 500+ | NNS* (95% CI) | 120 (76 to 193) | 88 (42 to 197) | 104 (56 to 203) | 513 (91 to 6536) | |
| (N (preventable cases)/N (cohort))† | (14/2326) | (5/643) | (8/1071) | (1/612) | ||
| % Preventable cases‡ | 1.6 | 0.6 | 0.9 | 0.1 | ||
The table presents effectiveness as the number needed to screen (NNS)—see below and Methods. Targeted screening is considered in subgroups stratified by age at primary care registration and WHO TB incidence in the country of origin.
*NNS, the number of registrations needed to be screened to identify one preventable case of foreign-born TB (95% CI) occurring within 5 years of Flag-4 registration. This was computed as the reciprocal of cumulative 5-year TB risk derived from Kaplan–Meier survival analysis (see Methods).
†Absolute number of preventable cases (N (preventable)) occurring in the observed cohort (N (cohort)). N (cohort) was the total number of immigrants in each specified subgroup observed for >6 months after Flag-4 registration that remained disease free during this period.
‡Preventable cases expressed as the percentage of all foreign-born TB. This figure summarises the likely impact on local case load of an effective latent TB infection screening programme in each stated subgroup.