18.4: Patient testing
18.4.1: HPA typing
Patients should be typed for HPA following the guidelines for donor HPA typing with the following exceptions:
- A provisional type can be issued on the basis of a genotype performed on one occasion. However, it is recommended that a second typing technique be used when quality exercises or routine practice have revealed technical problems when typing for particular polymorphisms. Typing of subsequent samples will allow a confirmed genotype to be reported.
- HPA typing of fetal amniocytes can be undertaken by molecular techniques using DNA isolated from non-cultured amniocytes and a provisional HPA genotype reported. The HPA genotype should be repeated on DNA extracted from cultured amniocytes and shown to be concordant with the first result.
18.4.2: Investigation of HPA antibodies
Patients should be investigated for HPA antibodies following the guidelines for donor investigation with the following exceptions:
- Laboratories serving populations with non-Caucasoid patients are advised to include cells in their panels which will aid the detection and identification of additional clinically significant antibodies (e.g. HPA-4, Naka/GPIV). If the acquisition of GPIV negative cells is not possible, an alternative approach is to establish assays capable of identifying GPIV antibodies that are controlled by appropriate positive control sera.
- Laboratories providing diagnostic testing for Neonatal Alloimmune Thrombocytopenia (NAITP) are advised to investigate cases with a clinical diagnosis suggestive of NAITP and a negative HPA antibody screen for antibodies against low-frequency or ‘private’ antigens. An effective approach is to use platelets from the child’s father as an additional panel cell (paternal platelets should be HPA typed as a ‘patient sample’). Alternatively, laboratories may refer such cases to a reference laboratory. In the event of a negative antibody screen in a case where NAITP is suspected and there is a potential HPA incompatibility between maternal and baby HPA types, laboratories are advised to repeat the antibody investigation 1 month after delivery.
- Laboratories providing diagnostic testing for platelet refractoriness should follow the algorithm for laboratory investigations of platelet refractoriness in Figure 16.1.
A patient with HPA antibodies should receive an HPA antibody card and, wherever possible, an information leaflet. However, before an HPA antibody card and information leaflet is issued, the patient should be typed and found negative for that antigen.