Fig. 1.

Examination of confounding effect of clinical variables in Cox regression models. Legend: Hazard ratios [HR] and 95% confidence intervals [95% CI] for risk of suicide associated with a diagnosis of post-traumatic stress disorder [PTSD], with various stages of adjustment during multivariable modelling. Results for unadjusted and adjusted models 1 and 2 are presented in Table 2. Adjustment 1 includes control for current age, natal sex, country of origin, population density and deprivation. Adjustment 2 includes additional control for previous diagnoses of psychiatric conditions before a PTSD diagnosis and parental severe mental illness [SMI], and shows substantial attenuation that can be attributed to one or more of these conditions. Adjustments 2a-2e show the risk of suicide associated with PTSD after adjusting for all variables in adjusted model 2, except the psychiatric condition stated. Models 2a-2d show that a diagnosis of major depression or anxiety disorder, bipolar disorder or non-affective psychotic disorder prior to a PTSD diagnosis or parental history of SMI do not explain attenuated suicide risk between adjusted models 1 and 2. Model 2e, however, shows that without control for previous non-fatal suicide attempts, the association between PTSD and suicide is stronger (HR: 4•23; 95% CI: 3•66–4•89), suggesting that previous non-fatal suicide attempts are an important confounder of the association between PTSD and later suicide risk. Adjustment 1a shows adjusted model 1 with additional control for previous non-fatal suicide attempts only (HR: 2•34; 95% CI: 2•02–2•71), demonstrating that approximately 57.1% of the excess risk of PTSD on suicide in adjusted model 1 was due to confounding by previous non-fatal suicide attempt.