Importance of high response rates in traumatic stress research.

Non-response and psychological resistance, i.e. degree of unwillingness to undergo the primary examination (screening), and its implications for estimation of PTSD prevalences was measured in a longitudinal study of 246 employees exposed to an industrial disaster (explosion/fire). Resistance including refusal, was measured by counting the number of contacts needed in the calling-in procedure to secure cooperation. Resistance to the primary examination related to severity of exposure (24.2% in the high exposure versus 6.8% in the medium exposure and 4.2% in the low exposure group). If those who initially resisted had been lost to the 7 month follow-up the total response rate would have been 82.8%, with an estimated frequency of high PTSS scores of 15. By increasing the response rate to 100%, the true prevalence of high PTSS scores increase to 22.4%. The initial resistance related strongly (P less than 0.001) to the severity of outcome at 7 months. The potential loss to the follow-up would have included 42% of the PTSD cases, and 64% of the severe PTSD cases would have fallen out, resulting in distorted prevalence rates of PTSD. The high potential loss to follow-up in the high exposure category would reduce the predictor value of belonging to that exposure group. The initial resistance in many who later developed PTSD was found to relate to the psychological defences such as avoidance against the re-experiences in the acute post-traumatic stress syndrome. For traumatic stress studies on the after-effects of shock traumas the implication of findings is that response rates need to be high. For primary and secondary prevention the implication may be that early outreach must be very active.