Chapter 7 Answers to self-check questions

7.1 Why are pregnant women considered as a special group with regard to the timing of sample collection prior to transfusion?

During pregnancy foetal red cells are found in small numbers in the maternal circulation. These may differ antigenically from the mother if they express antigens of paternal origin that the mother does not possess. These foreign antigens may stimulate an immune response in the mother. This is why currently after seven days the antibody screen on samples from pregnant women is not considered representative of the woman’s current immune status.

 

7.2 Why does the crossmatch not replace the antibody screen?

The red cells selected for antibody screening are homozygous for all the major clinically significant red cell antigens, whereas donor unit(s) may have only heterozygous expression of certain antigens. If the patient has a weak antibody against an antigen it should be detected at the antibody screening stage due to the stronger homozygous expression of the relevant antigen on the antibody screening cells. However, the patient’s weak antibody may not be detected in the IAT crossmatch if the antigen expression on the donor red cells is in the weaker heterozygous form. If the blood was transfused based upon the IAT crossmatch only a transfusion reaction can still occur even if the unit is heterozygous for the antigen.

 

7.3 What is the potential problem with performing the Abo and D testing on the same sample twice, and then issuing blood, where there is no historical grouping record for the patient?

Any laboratory error should be detected by testing the sample twice. However, if the sample was collected from the wrong patient then no matter how many times the sample is grouped the incorrect group may be obtained. Errors in patient identification and sample labelling occur but if the patient is bled more than once on different occasions there is a higher chance that the correct patient will be bled and therefore the right ABO and Rh D group obtained. But it is accepted that it is not always feasible to have samples taken on diff erent occasions.

 

7.4 Why is it preferable to use group A FFP for group B recipients and vice versa where ABO-identical FFP is not available?

Because plasma from donors of groups A and B generally have lower activities of ABO antibodies than plasma from group O donors.

 

7.5 What are the most common adverse transfusion reactions, and how are they caused?

Febrile non-haemolytic reactions (FNHTRs) are relatively common. They are defined as a 1°C or greater temperature rise with or without chills, and sometimes hypotension. They are probably caused by HLA or lymphocytotoxic antibodies, or cytokines released from white cells on storage, and so are much less common now that all blood components are leucodepleted before issue. Most symptoms are relatively mild and benign, and transfusion may usually continue if the patient’s temperature resolves with antipyretics such as paracetamol. Allergic (or urticarial) reactions are as commonly reported as FNHTRs and are caused by histamine release when the patient’s immune system recognizes an allergen in the donor plasma (or vice versa). This causes swelling and raised red welts that may be intensely itchy, and are treated with antihistamine. The majority of allergic reactions are mild and not life threatening, but they may occasionally be severe and may necessitate the provision of plasma-defi cient blood components to minimize the risk of reaction.

 

7.6 What are the initial laboratory tests to be carried out if a haemolytic transfusion reaction is suspected?

• Check for any clerical, collection, or administration errors that may have occurred.
• Check for signs of visible haemolysis in samples and component packs. Visual inspection of the patient’s plasma after transfusion and comparison with a pretransfusion sample is a sensitive method to detect intravascular haemolysis, as destruction of as little as 5 ml of transfused red cells will produce a reddish tinge indicative of haemoglobinaemia. Icteric (dark yellow/brown) plasma caused by hyperbilirubinaemia may indicate a haemolytic process that has been going on for some hours, or may be a sign of coincidental liver disease.
• A DAT should be performed on both the post-transfusion sample and the pre-transfusion sample, if available, for comparison. Where the DAT is positive, elution of antibody from post-transfusion red cells may aid identification, or confirm specificities of antibody detected in plasma or serum.
• The patient’s blood group and antibody screen should be retested with both the pre- and post-transfusion samples to ensure that they are the same blood group, and that a weak antibody has not been missed during pre-transfusion screening.
• Compatibility testing should be repeated using both the pre-transfusion and post-transfusion samples to ensure that all results were correct and to detect any antibody that may now be apparent post-transfusion.