8.1 Supplemental methods
8.1.1 Risk model calculations
The following equation was used to calculate the number of cases of each adverse event [AE], with and without PI:
\[ \begin{aligned} (\text{[AE] Cases without PI}) = &(\text{# whole blood donations})\times \\ &(\text{[AE] baseline risk})\times \\ &(\text{[AE] clinical outcome risk}) \end{aligned} \] \[ \begin{aligned} \text{[AE] Cases with PI} = &(\text{# whole blood donations})\times \\ &(\text{[AE] baseline risk})\times \\ &(\text{[AE] clinical outcome risk})\times \\ &(\text{[AE] transmissibility}) \\ \end{aligned} \] The annual cost of PI was calculated as:
\[ \begin{aligned} \text{PI cost per case} = &(\text{# whole blood donations transfused})\times\\ &(1 + \text{% donations not transfused})\times\\ &(\text{PI cost per donation.}) \end{aligned} \]
8.1.2 Estimation of malaria clinical outcome risk
The risk of clinical malaria infection from the transfusion of a parasitaemic donation was calculated from data in Allain 2016 [94]. In that study, the prevalence of parasitemia was 23% (50/217) in recipients and 25% (91/367) in donors. Parasitemia by malaria species was also reported:
- In donors: 56 had P falciparum only; 4 had P malariae only; 1 had P ovale only; 26 had both P falciparum and P malariae; and 4 had all 3 species.
- In recipients: 48 had P falciparum only; 1 had P malariae only; 0 had P ovale only; 2 had both P falciparum and P malariae; and 0 had all 3 species.
From these numbers, we calculated the following probabilities:
- Probability donation has non-falciparum species given that it has malaria: 38.46%
- Probability donation has non-malariae species given that it has malaria: 62.64%
- Probability donation has non-ovale species given that it has malaria: 94.51%
- Probability donation has neither P falciparum nor P malariae given that it has malaria: 1.10%
From these, the probability that a parasitaemic recipient who is transfused with a parasitaemic donation would receive a species they are not already parasaetimic for was calculated by taking a sum of the probability donations do not have each set of malaria species weighted by the probability that recipients do have each set of malaria species. The result was 37.47%.
The probability of clinical outcomes when a malaria-positive donation is transfused to a non-parasitaemic recipient was reported as 21.6% (8 of 37 transfusions) with a 95% confidence interval of 9.8–38.2%. We assumed that when a parasitaemic donation is transfused to a parasitaemic recipient, the risk of transmission is the same as with a non-parasitaemic recipient when the donation contained a malaria species for which the recipient was not parasitaemic, and the risk of clinical outcomes was 0 when the recipient was already parasitaemic with all malaria species in the donation. Therefore, the estimated clinical outcome risk for a parasitaemic recipient was \(21.60\% \times 37.47\% = 8.09\%\). We then calculated the overall expected probability of transmission of a malaria parasitaemic donation by weighting the probability in parasitaemic recipients (8.09%) and the probability in non-parasitaemic recipients (21.60%) by the proportion of recipients who were parasitaemic (23%) for an overall risk of clinical outcomes of 18.5%. For the uncertainty range, we assumed the value could range from 45% to 176% of the base case value based on the range in the confidence interval around the risk of transmission from Allain 2016.