Pre-transfusion testing- amanda Flashcards
Examples of pre-transfusion testing
ABO group
RhD typing
RBC antibody screen
RBC antibody identification
Crossmatching
Phenotyping
ABO and RhD methodology
Direct agglutination
What does the indirect antiglobulin test do?
Allows for detection of red cells IgG antibodies which would not cause agglutination in laboratory testing
Wha is indirect antiglobulin test used for?
- antibody screening and identification
- phenotyping
- crossmatch
What are the properties of red cells
-red cell membrane has negative charge which attracts positive ions from solution hence forming an ionic cloud around the red cells
-IgG antibodies cannot overcome the zeta potential between red cells which prevents them agglutinating together
Antibody screening
Patients serum or plasma is tested versus group O reagent red cells to identify the presence of antibodies other than ABO
- want to detect all clinically significant antibodies
- want to avoid nuisance antibodies
Indirect antiglobulin test employed
Needs to be sensitive but not necessarily specific
Why do we need antibody identification ?
IAT
Patients serum tested against a panel of reagent cells of known phenotype
Frequently includes an enzyme technique
Needs to be specific but not overly sensitive so as to avoid false positives
Extended phenotyping donors
Identification of suitable antigen negative units for patients with the corresponding antibodies.
Identification of suitable antigen negative units for regularly transfused patients who do not have corresponding antibodies e.g. sickle patients matched for Rh-C, -D, -C, -E, -e and Kell
Extended phenotyping patients
Confirms phenotype when antibody specificity identified
Aids in complex antibody investigations
Avoid sensitisation and antibody production.
What is crossmatching?
Testing of patients serum or plasma against potential donor red cells for transfusion by an IAT
Currently gold standard in compatibility test
Negatives of crossmatching
Time consuming
Resource heavy
Lead to delays in blood provision
Electronic issue requirements
-patient has confirmed ABO group, current IAT antibody screen negative, no history of antibodies detected
-recommended quality management system and laboratory processes in place
-LIMS control of the issue of blood components as recommended in the BCSH IT guidelines
When is EI acceptable ?
-current sample and history are identical
-no manual amendments have been made to automated results
-current antibody screen is negative
-patients G and S are complete and authorised by the LIMS
-patients doesn’t have previously known antibody of clinical significance
-patient is not excluded on clinical grounds
-current sample meets the sample timing and storage requirements
Risk of EI
Relis on a functional Trust IT network which requires a manual procedure in case of network down time
Report on introduction of EI and remote blood issue:
Median time to deliver blood urgently to cardiac unit reduced from 24 minutes to 59 seconds
Requests for blood which was subsequently not used reduced from 42% to 20% of all requests
Number of units issued reduced by 52%
Percentage of units issued that were transfused increased from 40% to 62%
Blood transfusion testing…
Manual methods are rarely undertaken but used in reference laboratories
Automated techniques with high throughput
Manual methods - tile and tube ABO/D typing, Tube IAGT
Low cost (initial outlay, maintenance, reagents)
Urgent release of test results
Suited to low throughput laboratories
Simplicity and familiarity
Established sensitivity and specificity
Flexibility
Manual testing advantages
Resolution of ABO anomalies
Resolution of RhD typing anomalies
Modifications to techniques e.g. changes to medium
Use of extended range of techniques, incubation temperatures to resolve complex serological problems.
Investigation of autoantibodies
Complex Antibody mixtures
Manual testing disadvantages
-not suitable for large batch testing
-elution of low affinity antibodies during washing phases
-variability in the red cell concentrations
-improper cell to serum ratio
-lack of consistency due to inter-observer variability-better results during the day than the evening shift or on-call
SHOT reports
-SHOT report- 75% of ABO grouping errors due to manual methods
-SHOT report- 5/6 grouping errors and all 9 near misses due to manual techniques
Advantages of automation
-improves objectivity and reproducibility
-reduces error potential
-documentation, traceability of tests, reagents and processes, and archiving of results
Disadvantages of automation
No one method unequivocally superior to another “ solid phase detects some antibody specificities more readily (anti–K,-Fya) and less for others (anti-E) ”
Urgent samples.
Initial set up costs.
Increased sensitivity can lead to detection of non specific and nuisance antibodies resulting in additional manual investigations.
Comprehensive staff training.
Back up system must be available if this is a manual technique then maintaining competency is an issue.
Uniformity of sample requirements.
Quality assurance and validation.
What’s the next big thing
Molecular testing
Arguments in support of the widespread introduction of molecular techniques
Molecular techniques – shift in emphasis
Genotyping superior to serology
Resolution of ABO and D variant antigens.
Identification of other polymorphisms
Genotyping can be used to type patients for whom serological techniques cannot be applied.
Multi-transfused patients
DAT positive
Recent HSCT
Lack of available serology reagents e.g. Colton and Dombrock
Donor mass screening for red cell and non red cell types simultaneously (HLA and HPA) as well as for rare types.
Molecular matching patients and donors to reduce the incidence of alloimmunisation.
Reduce requirement and therefore costs of serological testing.
Deducing RhD zygosity
Fetal blood group diagnosis of maternal blood.
More accurately typed panels e.g.MNS, Kell, Kidd Duffy, Lutheran, Dombrock, Cartwright, Colton.
Counter arguments for the universal introduction of molecular techniques.
Most transfusion centres in the Europe and the UK already routinely type their donors for Rh and K as a minimum, and currently for random patients this information is not accessed.
Genotype does not always reflect phenotype
Identification of genetic polymorphisms of unknown clinical relevance.
Dry matching “myth” O R2R2 Fy(a+b-), Jk(a+b-) ss units 1:20,000, when do we drop the matching criteria?
Urgent requests.
Over requesting due to very specific requirements.
Serology must be kept so no strong compulsion to change to molecular techniques.
Costs could be reduced by centralisation of testing but all IT systems would need to be linked to maintain donor and patient genotype information.
Future developments
Removal of the need to provide ABO compatible blood……
The first approach relies on enzymatic conversion of specific blood group antigens, i.e. manipulation of the ABO system.
Non-specific camouflage of antigens with polyethylene glycol (PEG) derivatives.
In vitro production of red blood cells with a pre- defined antigenic profile from genetically manipulated stem cells
