The Cost of Costing Treatments Incorrectly: Errors in the Application of Drug Prices in Economic Models Due to Differing Patient Weights

  • Hatswell A
  • Porter J
  • Hertel N
  • et al.
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Objectives: Drug costs are generally a key driver of the results of economic models. We tested the impact on drug cost estimates for the following common approaches: using mean patient weight, individual patient weights or fitting a distribution to the observed patient weights. Methods: For the analysis, we utilised patient weight and height data from trial CA184-024 (517 patients) in metastatic melanoma. Based on this dataset, costs of a single administration of drug therapy were calculated using UK list prices. Costs were calculated for four recently licensed treatments with different posologies: ipilimumab (mg/kg, with 2 vial sizes), cabazitaxel (mg/m2), ustekinumab (doubled dosage over 100kg patient weight) and romiplostim (μg/ kg, with a large, single vial size). Results: Cost estimates using the mean patient weight were £18,750, £3,696, £2,147 and £482 per administration of ipilimumab, cabazitaxel, ustekinumab and romiplostim, respectively. These results increased by 4.9%, 2.3%, 10.3% and 36.6% when costing individual patient weights, and by 5.2%, 2.3%, 11.8% and 36.9% when fitting a distribution to the patient weights. The use of only mean patient weight consistently underestimated costs compared to methods that incorporated the distribution of weight data. Sampling from the observed patient weight distribution provides a more accurate estimation of costs; however, it is subject to over- or under-estimation, depending on enrolment in a trial programme, particularly amongst patients who are substantially over- or under-weight. Conclusions: Accurate estimation of drug costs requires an understanding of the distribution of patient weights. Failing to take this into account can result in cost estimates that are substantially lower than will be seen in practice, which could (in turn) impact treatment (implementation) decisions. These errors would be further compounded should drug wastage not be adequately captured. Modellers should be mindful of these issues when costing therapies or conducting health technology assessment submissions.




Hatswell, A. J., Porter, J., Hertel, N., & Lee, D. (2014). The Cost of Costing Treatments Incorrectly: Errors in the Application of Drug Prices in Economic Models Due to Differing Patient Weights. Value in Health, 17(7), A323–A324.

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