Gestational diabetes mellitus (GDM) is the most common medical complication during pregnancy and is defined as carbohydrate intolerance with varying levels of severity, with the onset or first recognition occurring during pregnancy. A variety of different tests and guidelines have been used to screen for GDM over the past decade and due to this the prevalence’s for GDM that are reported in studies tend to vary considerably. However, in Scotland there has been controversy over the method for the screening and the diagnosis of GDM, reflecting the lack of consensus for the diagnosis of this condition. This thesis therefore undertakes a cost-effectiveness analysis that compares four screening test strategies that use various combinations of screening and diagnostic tests with a strategy that involves no screening.The first objective was to explore the economic approach to evaluating diagnostic testing in GDM. In consultation with experts and informed by comparable diagnostic testing models, the thesis adapted a conceptual model to the practice of GDM detection. The thesis found that the most appropriate model makes use of a combination of tests as either “negative dominant strategy” (NDS) or “positive dominant strategy” (PDS), allowing the clinician to consider test results in terms of the differences in false negative (FN) and false positive (FP) test results, as combining the tests in terms of NDS and PDS involves a tread-off between sensitivity and specificity. A key input parameter of the model was identified to be the disease prevalence. However, due to limited known evidence in Scotland, the second objective was to assess the evidence on this key parameter. A systematic review was conducted to evaluate the prevalence of GDM, considering not just screening test characteristics, but also population characteristics. The review explores the possibility that variations in over half of the studies can be explained by ethnicity and diagnostic screening strategies and whether 75g or 100g oral glucose tolerance test (OGTT) methods of testing for GDM can cause variations in the results. Mothers with risk factors are prone to testing positive which in turn leads to higher prevalence rates compared to other populations and screening ethnic groups that have a high risk of developing GDM can indeed result in high prevalence estimates, ranging from 8.5% to 12.8%.Decision making in healthcare over the past decade has increasingly been based on considerations of cost effectiveness, including national guidelines for GDM screening. The third objective was to summarize and appraise the present economic evaluation literature. Thus, a systematic review of economic evaluations, as outlined in the various cost and cost-effectiveness studies that have been published in recent years, was performed to critically appraise the current analytical methods used to measure the cost-effectiveness of screening tests in order to develop a standardised economic model. Costs associated with the screening and management of GDM vary widely by country, ranging approximately from £2.42 to £50.9 per case detection, and are dependent on the tests used, the screening approach, how the costs are calculated and the prevalence of GDM in the population. The review of the CEA studies has significant implications for future research and policy making and as such long term consequences are appropriate outcomes for the CEA of screening tests for GDM in order to capture all long term adverse health outcomes for GDM. Therefore, the economic evaluation for GDM should account for the effectiveness of postpartum screening for type 2 DM. The aforementioned conceptual model and the results of the systematic reviews of diabetes prevalence and GDM screening cost-effectiveness have been synthesised into a model to estimate the cost-effectiveness of screening for GDM based on four screening guidelines that include 1) SIGN 2001 (random plasma glucose followed by 75g OGTT) , 2) NICE 2008 (risk factors screening followed by fasting plasma glucose and 75g OGTT) , 3) Consensus 2010 (75g OGTT) and 4) SIGN 2010 (risk factors screening followed by 75g OGTT) versus 5) no screening. This probabilistic model was used to estimate and compare the costs and quality adjusted life years (QALYs) of screening tests for GDM. Three independent decision trees, for case detections, short term complications in the first year and long term complications over the lifetime, explored and considered the combinations of screening and diagnostic tests in terms of NDS and PDS. Independent decision trees would allow policy makers to focus on each part of the model separately. The primary outcomes of the analysis were the incremental cost per case identification, one year QALYs for short term complications and lifetime QALYs for type 2 diabetes mellitus for long term complications. Case identification was insufficient for policy makers because it fails to take account of the consequence of false positive and false negative results of tests.For short term complications, the incremental cost-effectiveness ratio was £46,760 per QALY for the two-step approach with SIGN 2001 (NDS) using 75g OGTT to confirm any positive random plasma glucose (RPG) before treatment compared with no screening. At willingness to pay £30,000/QALY, this strategy has 64% probability of being cost-effective for short term complications. The cost effectiveness of screening tests for GDM, to prevent short term complications, is dependent on the probability of GDM being undiagnosed. Additionally, treatments during gestation are important as they reduce additional costs that may be required to treat serious adverse complications.PDS screening where all pregnant women with one or more high risk factors are requested to undertake 75g OGTT diagnostic test, proposed by SIGN 2010, is the most cost-effective strategy in long term complications. SIGN 2010 (PDS) has higher QALY (80.9736) and is less expensive (£4,088) than the other strategies and dominates the other screening test strategies for long term complications. At a threshold of £30,000/QALY, the CEA illustrates that the probability that SIGN 2010 (PDS) will be cost-effective is approximately 55.8%. The cost effectiveness of screening tests for GDM, to prevent long term complications, is dependent on the probability of GDM being over diagnosed. If mothers have received previous diagnoses of GDM, this should trigger regular screening for type 2 DM so that it is discovered early on, before the onset of symptoms or the development of complications associated with type 2 DM. Postpartum screening and the subsequent treatment of GDM presents an important opportunity to reduce type 2 DM. This thesis provides the first economic model of a screening test using independent decision trees, split by NDS and PDS. By using NDS and PDS, decision makers can interpret the combination of test results. This better presents the consequences of false positive and false negatives and a trade-off between sensitive and specificity. The thesis finds novel value of applying this methodology to GDM screening. By using independent decision trees for NDS and PDS, the model was able to identify long term complications as the most important factors affecting the results of screening test strategies. Currently, all guidelines for GDM screening tests which also included two Scottish guidelines SIGN 2001 and SIGN 2010 are performed as NDS with regard to both screening tests and postpartum screening. Thus, in Scotland, policy makers or clinicians should consider SIGN 2010 with PDS for screening tests for GDM in order to prevent long term complications. Lastly, this work is also the first to apply the expected value of information (EVPI) to the area of GDM screening. The population EVPPI of £784,042 for long term complications shows that there is greater uncertainty with respect to long term complications and that collecting information on long term complications is likely to be worthwhile.Thus, it captures the long time horizons in screening programmes required for decisions about the value of further research and the expected payoff of conducting further research to resolve the model uncertainties.
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The economics of diagnostic test: the cost-effectiveness of screening test for gestational diabetes mellitus in Scotland
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