Screening contacts of patients with extrapulmonary TB for latent TB infection has a high yield in northwest London

The 2016 TB National Institute for Health and Care Excellence (NICE) guidelines1 recommend that adults aged 18–65 who are close contacts of patients with pulmonary TB (PTB) or laryngeal TB have screening for latent TB infection (LTBI) and hence imply that contacts of extrapulmonary TB (EPTB) need not be screened. This is consistent with US guidelines, which similarly recommend screening only contacts of PTB and laryngeal TB.2 At our TB centre in northwest London, consistent with 2011 NICE recommendations,3 we routinely screen close contacts of all our patients with TB regardless of their site of disease.

We are concerned that the suggested change to our screening policy could mean that an opportunity to diagnose and treat LTBI will be missed. The Greater London area accounted for 39.4% of all UK cases and had a prevalence of 26.2 per 100 000 in 2015.4 London Northwest Healthcare NHS Trust has one of the largest TB units in the UK, and we serve areas where the TB incidence is up to 99.5 per 100 000.5 Our area has a high ethnic diversity with a high proportion of non-UK-born residents and recent immigrants.

We audited the yield of our screening policy. We identified all patients who we treated for TB during 2015. The patients with TB and their contacts were identified from our database. We screen, in the conventional way, using Mantoux test, symptom review and chest X-ray.

In 2015, we treated 189 cases of TB and identified 698 close contacts (3.7 contacts per TB index case). In 2015, we screened fewer contacts older than 35 than we do currently because we were not offering LTBI chemoprophylaxis to those aged >35 years due to the risk of hepatotoxicity;3 thus, the number of contacts screened per case will likely rise in future. The results are summarised in table 1. In total, 98/189 (51.9%) of our patients had EPTB. Of all the contacts we screened, 1.3% had active and 19.9% had LTBI. Among contacts of EPTB index cases, 0.3% had active TB and 10.4% had LTBI. In comparison, among contacts of PTB index cases, 2% had active and 27.1% had LTBI.

There are few recent data on the screening outcomes of EPTB.6 We consider our detection rate of LTBI is a combination of recent and distant exposures to Mycobacterium tuberculosis, the latter being high because most of our cases and their contacts are non-UK born. Screening close contacts of EPTB cases represents a direct way to access individuals at high risk of TB/LTBI even if infection has not necessarily been contracted from the index case. Provided the patient with LTBI adheres to treatment, treatment reduces the risk of subsequent TB by ∼90%.7

In comparison with other screening programmes in the UK, the overall active or LTBI detection rate of 21.2% and the active or LTBI case identification rate of 10.7% among EPTB contacts is favourable. The detection of one active case among 299 screened contacts of patients with active EPTB also compares favourably to other screening strategies for active TB.8 By comparison, screening for abdominal aortic aneurysms over the age of 65 identified <1% of men eligible for yearly surveillance,9 cervical cancer screening identifies ∼7% of those with abnormal smears10 and bowel cancer screening identified ∼2%.11

We therefore propose to continue to screen the contacts of both our patients with PTB and EPTB. We recommend other TB units audit their results to decide what is appropriate for their patients.