A decision tree was used to represent patient pathways and combine the evidence from published literature, to simulate the outcomes, for patients with minor head injuries, aged 40 or 75 years, who presented to the emergency department. A lifetime horizon was specified and the authors stated that the perspective was that of the UK NHS and Personal Social Services in England and Wales.
The sensitivity and specificity of the diagnostic strategies were from a large, unselected cohort validation study. The primary measure of patient outcome was their Glasgow Outcome Score (GOS), and the data were from several sources depending on the strategy and pathway. For patients with a neurosurgical injury, who received prompt treatment, a meta-analysis of studies identified by a systematic review was undertaken. For those, who received delayed treatment, and for those with a non-neurosurgical injury, where the lesion was detected on CT and the patient was admitted and appropriately treated, one relevant study was used. For those with a non-neurosurgical injury, where CT was not performed and the patient was discharged home without appropriate treatment, an assumption was needed due to a lack of evidence; it was assumed that the delay had a similar effect to that in the treatment of neurosurgical injuries, and the outcomes were adjusted accordingly. The mean life expectancy was from UK life tables.
Monetary benefit and utility valuations:
A literature review was undertaken to identify studies that estimated the utility for each of the GOS states. The most relevant study was selected, based on its compliance with the NICE reference case methods. This was a study of long-term GOS outcomes and health-related quality of life, that used the EQ-5D questionnaire, with 87 patients who underwent CT for head injury.
Measure of benefit:
Quality-adjusted life-years (QALYs) were used to produce the ratio of cost per QALY. The utilities were discounted at an annual rate of 3.5%.
The costs were the direct costs of diagnostic management, which included investigations; CT scan; hospital admission and subsequent neurosurgical treatment; intensive care; nursing home care; rehabilitation for the severely disabled; and the treatment of glioma. The hospital costs and nursing home costs were UK Department of Health National Reference Costs. The long-term costs of care and rehabilitation were based on expert opinion. The rehabilitation costs were from the UK Personal Social Services Research Unit (PSSRU), and the costs of glioma were from one study. The costs were in UK £ and were discounted at an annual rate of 3.5%.
Analysis of uncertainty:
The authors used univariate sensitivity analysis to explore the impact of varying each parameter to the extremes of its confidence interval, or altering the discount rates for the costs and benefits to zero or 6% per year. Probabilistic sensitivity analysis was undertaken to explore the impact of the joint uncertainty in all the parameters.