In this week’s case we investigated someone with a panreactive antibody screen. In simple terms, this means that something in the patient’s plasma is reacting with all of the red cell reagents we use in our panel.
This situation causes concern in the transfusion lab because, whatever the cause, the patient could have an underlying alloantibody which we cannot see, which could then cause a transfusion reaction if we transfuse blood to the patient with the corresponding antigen.
However, as always, no patient should come to harm due to a lack of blood. Further laboratory tests will take time, and it is better to deal with a haemolytic transfusion reaction in a living patient rather than having the perfect blood for a corpse!! If you need blood – the lab will supply the ‘least incompatible’ blood they have with the available results.
In our case, there was not an emergency need for blood, and so we can investigate further. Below are some general points to consider.
- Clinical context is hugely important (when is it not?!) – key things to cover are:
- Is there clinical or laboratory evidence of haemolysis to support an autoimmune haemolytic anaemia (anaemia, high unconjugated bilirubin, reticulocytosis, low haptoglobins, blood film showing spherocytes, polychromasia)
- Is the patient on any relevant drugs, or have they had some treatment recently (IVIg, monoclonal antibodies including daratumumab)
- If the patient is female, what is their obstetric history?
- Has the patient had a previous blood transfusion? How recently?
- Tests the transfusion lab will consider:
- DAT (direct antiglobulin test)
This detects whether there are antibody or complement coated RBCs in vivo ie in the patient’s circulation. This could be an autoantibody, an alloantibody to previously transfused RBCs or a drug effect. Remember – in the context of a panreactive antibody screen, a positive DAT does not exclude a clinically important alloantibody, it suggests there is ‘something’ else there to get rid of before we can be confident in our alloantibody screen
Using a reagent (called ZZAP – dithiothreitol and papain), we can remove autoantibody from the patient’s own red blood cells. You can then perform the antibody screen on the ‘clean’ sample to seek underlying alloantibodies. Note – if the patient has been transfused in the past 3 months, alloabsorption techniques are required, using selected reagent RBCs to remove alloantibodies
- Red cell eluate
If there is a history of recent transfusion or absorption studies are inconclusive, elution can be performed. An acid is used to remove antibody which is coating the surface of the patient’s red cells. The eluate refers to the plasma which is left behind (and so will contain any antibody to then be tested against reagent red cells)
Daratumumab is an anti-CD38 licensed for treatment in multiple myeloma. Daratumumab causes significant issues in transfusion testing, because CD38 is expressed on red blood cells. This means that the patient plasma sample used for the indirect antiglobulin test (antibody screen) will contain daratumumab, and so will coat all of the reagent RBCs, causing a universal positive result ie. panreactive antibody screen (see image below). This can persist up to 6 months from completion of daratumumab therapy.
To get around this problem, reagent RBCs can be treated with DTT (dithiothreitol) to remove the daratumumab and allow antibody screening to proceed. DTT also removes the K antigen (from the Kell system) from the RBCs, and so you cannot establish the patients K status (unless they have had a sample processed pre-daratumumab). Therefore, if their K status is not known, give K-ve blood.
NHSBT have guidance on testing required for patients before they start daratumumab therapy to get around this issue:
- ABO and D group and antibody screen
- Direct antiglobulin test (DAT)
- Extended phenotype (Rh CcDEe, MNSs, Kk, Jka and Jkb, Fya and Fyb)
- If transfusion is required, give ABO, extended Rh and K compatible units (to limit the risk of alloantibody formation which may subsequently be difficult to elicit)
As always, communication is key – between the clinical team, transfusion lab and patient (they should have a card stating they are on daratumumab)
NHSBT clinical guidelines: Managing patients on monoclonal antibody therapies:
NHSBT clinical guidelines: Investigation and clinical management of patients with a positive DAT with and without haemolysis:
Blooducation podcast: Investigation of the patient with a panreactive antibody screen:
Blood Bank Guy podcast: Daratumumab Effect: