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Adams et al cast doubt on the cost‐effectiveness of opportunistic chlamydia screening, as implemented in the English National Chlamydia Screening Programme.1 A recent economic evaluation of a proactive postal screening approach has shown that this is also unlikely to be a cost‐effective approach.2 These two studies, using state‐of‐the‐art individual‐based transmission dynamic network models,2,3 contrast sharply with almost all other published evaluations. Their results contradict not only those using an inappropriate modelling approach,4 but also more recent studies that used dynamic modelling and found screening to be cost effective5 or even cost saving.6,7
Adams et al's study is an important contribution to the debate about the appropriateness of chlamydia screening programmes.8 They clearly show that a chlamydia screening intervention can appear cost effective or not, depending on the assumption made about the probability of endocervical chlamydia progressing to pelvic inflammatory disease. They concluded that opportunistic screening was not cost effective if the progression rate was below 10% and that available epidemiological data were incompatible with a higher progression rate. The recent economic evaluations using individual‐based dynamic modelling that found screening to be cost saving used figures of 20% to 25%.6,7
Decision makers such as the National Institute for Health and Clinical Excellence (NICE) use the incremental cost‐effectiveness ratio (ICER) for decision making. In this case, the ICER refers to the additional cost per additional unit of benefit of screening compared to the alternative of no screening. Unfortunately, Adams et al misleadingly refer to the results of their main evaluation and the sensitivity analysis as average cost‐effectiveness ratios (average CERs). Their definition, “(difference in costs)/(difference in benefits) between screening and no screening” is, however, the standard definition for the ICER.1,9 The standard unit of benefit is the quality adjusted life‐year (QALY) which is a single measure summarising health improvements resulting from changes in both quality and quantity of life. The only QALY estimates for chlamydia and its complications were derived from an expert panel meeting rather than primary research,10 so their validity is not known.4 Most economic evaluations of chlamydia screening have therefore estimated the costs of screening per major outcome averted, typically including pelvic inflammatory disease, ectopic pregnancy, tubal infertility and neonatal complications.2,5,6,7 Such studies can be compared among themselves, but are less useful for decision making because there are no agreed thresholds for cost‐effectiveness measured in “natural units” such as these.
Adams et al used both measures of cost‐effectiveness. They estimated that the current strategy being implemented in the National Chlamydia Screening Programme in England had an ICER of £27269 per QALY gained and £1348 per major outcome averted, compared to no screening, assuming about 40% of sexually active women and men under 25 years would be screened every year, and that 10% of untreated chlamydia cases result in pelvic inflammatory disease. They suggest that such a programme could be accepted on cost‐effectiveness grounds because the ICER was below £30,000 per QALY. In fact, NICE guidance states that “Above a most plausible ICER of £20,000/QALY, judgements about the acceptability of the technology as an effective use of NHS resources are more likely to make more explicit reference to factors including: the degree of uncertainty around the calculation of the ICERs …” (section 22.214.171.124; page 33).11 Given the considerable uncertainty about the QALY estimates used by Adams et al this ICER, in QALY terms, exceeds the acceptable threshold approved by NICE and should be subjected to much greater scrutiny. Furthermore, the coverage of chlamydia screening assumed in the baseline model has not been achieved in any area of England that has implemented the screening programme. If coverage were to reach 10% of both women and men every year, the ICER would be £57000 per QALY.
The clear uncertainty about the economic benefits of organised chlamydia screening programmes reinforce doubts about their long‐term effectiveness.8 It seems paradoxical for Adams et al to suggest that the priority for research is to determine the progression rate of chlamydia to pelvic inflammatory disease after a screening programme has been introduced. Adequate understanding of the natural history of the condition is a basic prerequisite to introducing a screening programme.12 Until this happens, however, the doubt and debate will continue.