Signature
C_drug = C_planned * RDI
| Inputs | Definition | Unit |
|---|---|---|
C_planned | Drug cost if every planned dose were given on time | pounds per cycle |
RDI | Delivered dose intensity divided by planned dose intensity | proportion |
C_drug | Drug cost per treatment cycle reflecting the dose actually given | pounds per cycle |
|---|
Function
Dose-response curves and the dose-dependent drug cost behind incremental cost per extra responder
Links the expected response to the dose through the sigmoid Emax curve and the dose to the drug cost actually paid, through whole single-use vials and relative dose intensity, so that doses can be compared by the incremental cost per extra unit of effect. The incremental ratio between adjacent doses is HE-FM-ICER-001 with the sequential rules of HE-FM-CEA-001 and HE-FM-DOM-002, and the average ratio against placebo HE-FM-ACER-002. Notation follows the Dose Response article.
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Implementations
Excel
Drug cost after relative dose intensity from named cells
With PlannedCost and RelDoseInt named, the formula returns the modelled drug cost, held in DrugCostRDI.
=PlannedCost*RelDoseInt
Assumptions
Drug cost in proportion to the dose delivered under RDI scaling
The scaling assumes the drug cost falls in proportion to the dose; under whole-vial pricing a reduced dose saves money only when fewer vials are opened.
RDI from the trial that supplies the efficacy
Trial efficacy already reflects the doses received in the trial, so an RDI from the same trial lowers cost without changing effect; an RDI from another source breaks that link.
Worked examples
Course at 150 mg with vial sharing and an RDI of 0.90
A planned course costing 5,400 pounds with vial sharing costs 4,860 pounds at an RDI of 0.90 (computed here for illustration).
C_planned = 5400; RDI = 0.9; C_drug = 4860
Planned cost at the lower end of the RDI range in NICE appraisals
At an RDI of 0.85, the lower end of the 85 to 100 per cent range Dasgupta and colleagues report, a planned cost of 600 pounds per administration becomes 510 pounds (computed here for illustration).
C_planned = 600; RDI = 0.85; C_drug = 510
Common errors
Applying RDI to a per-mg cost under whole-vial pricing
A 150 mg administration at an RDI of 0.90 delivers 135 mg, which still needs two 100 mg vials at 600 pounds, while the per-mg scaling gives 540 pounds, so the saving is overstated (computed here for illustration).
Leaving the RDI calculation undocumented
Dasgupta and colleagues found RDI or a dose delay factor in 63 of 265 NICE solid-tumour appraisals, with RDI from 85 to 100 per cent and widely varying methods; excluding the RDI adjustment generally raised ICERs by 5 to 10 per cent, and committees frequently questioned the transparency of RDI calculations and the assumptions on delayed or missed doses and wastage.
Sources
Relative dose intensity as delivered over planned dose per unit time
Gadisa DA, Assefa M, Tefera GM, Yimer G. Patterns of anthracycline-based chemotherapy-induced adverse drug reactions and their impact on relative dose intensity among women with breast cancer in Ethiopia: a prospective observational study. Journal of Oncology. 2020;2020:2636514. doi:10.1155/2020/2636514 (abstract read). Abstract (Background): relative dose intensity is the percentage of actual dose received per unit time divided by planned dose per unit time, with values below 85 per cent regarded as below optimal.
RDI in NICE appraisals of solid tumour treatments and its effect on ICERs
Dasgupta A, Kaushik A, Pandey S, Attri S, Miller P, Tolley K. A review of the utilization of relative dose intensity and dose delay factor in health technology appraisals of oncology drugs in solid tumors. Advances in Therapy. 2025;42(11):5313-5340. doi:10.1007/s12325-025-03358-6 (abstract read). Abstract (Results): of 265 HTAs sourced to 31 March 2024, 63 included RDI or dose delay factor variables; RDI ranged from 85 to 100 per cent across all HTAs; approaches to calculating and applying RDI varied widely; committees frequently raised concerns about the transparency of RDI calculations and assumptions on delayed or missed doses and wastage; excluding RDI adjustments generally increased ICERs by 5 to 10 per cent.
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