Background Chest radiographs are commonly used for the diagnosis of tuberculosis and to assess the extent of disease. A relationship between the extent of disease as determined by smear grade and cavitation has been shown to predict 2-month smear results but little has been done to determine whether radiological severity reflects the bacterial burden at diagnosis.

Design/methods Pre-treatment chest X-rays from 1837 subjects with smear positive pulmonary tuberculosis enrolled into the REMoxTB trial were reviewed retrospectively. Two clinicians blinded to clinical details using the Ralph et al scoring system¹ (comprised of the percentage of affected lung field and presence of cavitation) performed separate readings. An independent reader (a radiologist) reviewed discrepant results for quality assessment and cavity presence. The final cavity assessment was combined with the average percentage affected value to give a severity score (maximum score 100% + 40 if any cavitation). The Ralph score was compared to the time to positivity (TTP) of liquid cultures as measured by MGIT 960. The association between CXR severity score and time to positivity (log scale) was evaluated using Spearman’s rank correlation coefficient. A Welch t-test was used to compare TTP of those with high and low disease extent and those with and without cavitation.

Results Matching sets of data were available for 1422 subjects. The median severity score was 53.75/140 (IQR 32.03–66.25) and median time to culture positivity 117 h (4.88 days). CXR severity score was weakly correlated with time to positivity (Spearman’s correlation -0.20, p < 0.0001). Time to positivity was higher in those without cavitation (difference 23.7 h, p < 0.0001) and those with a low area affected (difference 12.1 h, p < 0.0001).

Conclusions The radiological severity of pulmonary tuberculosis at diagnosis is weakly correlated with bacterial load as measured by TTP. This suggests that, in addition to bacterial burden, other factors such as immune response influence radiological appearances.

References

1. Ralph AP, Ardian M, Wiguna A, et al. A simple, valid, numerical score for grading chest X-ray severity in adult smear-positive pulmonary tuberculosis. Thorax. 2010;65:863–9