Platelet Serology and Transfusion Flashcards
What patient cohort is most likely to get platelet transfusion
Leukaemia patients
What platelet count would you be concerned with
Less than 150 if not a leukamia patient
Why are leukaemia patients so highly transfused with platelets
Their counts tend to not or barely improve after transfusion, will need multiple tranfusions to increase prior to surgery etc
What antigens are found on platelets
ABO (5-10%) high expression of A or B
HLA class I
HPA antigens
Lewis, I, P, Cromer, DAF
If LD they will have less class II due to less leucocytes
Why do we universally irradiate platelets?
To prevent GVHD
This doesnt affect platelets unlike rbcs
Are platelets typed
Yes as though to be a rbc product
What kind of antigens are found on platelets
Cold, carbohydrate antigens
Talk about HPAs, what are they, how were they discovered
Human platelet antigens
We dont know the number of these antigens
First discovered in investigation of NAIT
Located on platelet membrane glycoproteins
These GPs are involved in haemostasis, through integrated with proteins in the endothelium and coagulation proteins
How were HPAs originall named
They were originally named after antibody markers
There is a lot of overlap between groups
ISBT established HPA nomenclature in 1990
Systems are bi-alleleic with each allele being co-dominant
How are HPAs named, what is the significance of this
Each system numbered consecutively: HPA1, HPA2 etc
High frequency antigens = a, low = b etc
Thus any a antigens are hard to get platelets for
What are the most frequently detected antibodies
anti-HPA-1a
anti-HPA-5b
anti-HPA-15b
NB: 1a and 5b most associated with neonates
Where is HLA typing done?
Bristol
What are the three roles of different platelet antigens?
Collagen binding
Fibrinogen binding
vWF binding
When might you see antibodies against HPA
In certain conditions where there are issues with poor binding to collagen or fibrinogen etc -> they will lack certain HPA antigens and thus produce antibodies against them
These can be hard to get platelets for
Where are the majority of HPAs located
On the GpIIb/IIIa complex
What is the role of GpIIb/IIIa complex?
Central role in platelet aggregation
Role as a receptor for fibrinogen and fibronectin
List some of the platelet membrane glycoprotein complexes
GpIIb/IIa
Gp Ib/IX/V
GP Ia/IIa
GP IV
Talk about GP Ib/IX/V
Receptor for vWF
deficient in Bernard-Soulier syndrome
Talk about GP Ia/IIa
Involved in adhesion to collagen
In who is GPIIb/IIIa deficient in
Deficient in Glanzmann’s Thrombasthenia
In who is GP IV deficient in?
3-5% of Black and Asian individuals
What are the different platelet products
Pooled platelets
Apheresis platelets
NB: all are irradiated
How are pooled platelets made
Made from top and bottom packs
Platelet rich plasma/buffy coat of four donations are pooled together
If one unit is Rh+ then whole pool is Rh+
Platelet additive solution
LD, Irradiated and BacT
What is the conc of apheresis vs pooled platelets
45-85 x 10^9 pooled
3.0 x 10^11 apheresis
How are platelets stored
@22 degrees
Maintained pH of 6 or above
What is the ratio of PAS to palsma in pooled units
75% PAS and 25% plasma
What donors do we use for plasma and why?
Male only
No HLA antibodies
Avoid Trali
Since when have we been irradiating platelets and why?
Since 1998
Reduce FNHTR
Reduce refractoriness
Talk about apheresis donation
Takes 60-90 minutes
Can donate every 28 days
Can donate up to 3 splits at once
Platelets are removed and other components are returned to the donor
LD as part of process
Storage as per pooled platelets but in plasma not PAS
What are some alternatives to PAS
Electrolyte solutions containing acetate, citrate, phosphate and chloride
For how long are platelets stored and in what
Constant controlled agitation with gaseous exchange
Polyolefin packs
7 day storage (used to be 5 days)
When are platelets indicated?
Haemorrhage prophylaxis in thrombocytopenia or platelet function defects e.g. BMT
Bone marrow failure - prophylactic measure
Chemotherapy or irradiation of bone marrow impairing plt production
Immune thrombocytopenia -> depending on type
Before surgery if low
Acute DIC if bleeding
Inherited defects
Dilutional thrombocytopenia following massive transfusion
Auto-immune thrombocyoptenia - depend on type and if bleeding
Why do you need to be careful when issuing platelets in a thrombocytopenia
Should first establish cause of thrombocytopenia as plts not always indicated and are sometimes even contraindicated
Audits in the past suggest the over-prescribing of plts in these scenarios
Talk about platelet transfusion for bone marrow failure
Given prophylactically
If count of 10x10^9/L then transfusion indicative
In BMT we try to always keep count over 10
Seen in chemo or ablation
But if bleeding then as indicated
Talk about platelet tranfusions for surgery
50 x 10^9 or less will require tranfusion
Very hard to get these counts up to 100 so instead we try to maintain 50
But for normal patients cut off is 100
Talk about platelets in DIC
Only given if bleeding with severe thrombocytopenia
If you give platelets in the coag phase you will be adding fuel to the fire etc
What inherited defect required platelet transfusion
Wiskott-Aldrich Syndrome
Talk about refractoriness and anticipation of refractoriness
Anticipation of refractoriness in BMT or bleeding patients
Refractoriness is seen in a majority of these patients
It is more of a guide for when you should investigate HLA antibodies
When are platelets not indicated
HIT
Chronic DIC
What is refractoriness?
Repeated failure to respond satisfactorily to platelet transfusions and is common in multi-transfused patients >50%
What are the common causes of refractoriness?
Anti Class I HLA antibodies or anti HPA
Platelet consumption e.g. DIC or drugs or sepsis or bleeding
Spleen is seen to consume many platelets in inflammation or trauma
Define refractoriness
24 hour platelet increment <5 x10^9/L with two or more consecutive transfusions
How is platelet refractoriness treated?
Requires the identification of antibodies and selection of a ‘matched’ donor e.g. through apheresis
NB: matched platelets cost double the price, as the donor will have to be asked to come in
Were looking at a prophylactic anti-HLA a1 using MABs but havent been successful yet
IVIG can reduce the amount of Ig transfer across the placenta -> occupies binding sites in placenta for Abs to cross-over
IUT with compatible platelets
Corticosteroids
What is NAITP?
Neonatal alloimmune thrombocytopoenic Purpura (NAITP)
Platelet equivalent of HDFN
How frequent is NAITP
1 in 1500 pregnancies are affected
50% occur in the first pregnancy
Usually a sister or a mother will have produced the same antibodies during pregnancy
Same frequency as HDFN really
Otherwise theres no way of knowing until baby is born with lots of bruising
What are the common causes of NAITP?
HPA-1a most common
HPA-5b second most common
-> platelet antigens are expressed at about 16 weeks
-> Ab transferred across placenta at 14 weeks
Talk about severity of NAITP
Babies born with bruising
Cerebral haemorrhage most severe outcome
Severity based on affinity of IgG
Clinical features of NAITP
Purpura/bleeding
Haemorrhage
Hydrocephalus
Severity associated with plt count <20 x 10^9/L
What HLA is associated with increased severity of NAITP?
HLA-DRB3*0101 allele
These are 140 times more likely to make anti-HPA-1a
Talk about Post Transfusion Purpura
1:500,000 risk, very rare condition
Anamnestic response occuring approximately 1 week post-transfusion
Mostly caused by anti-HPA-1a but can cross react with patients own platelets
produce an ab against transfused platelets which then react against your own
Can cause severe bleeds, plt counts <10 x10^9/L with purpura and haemorrhage
Abs tend to cross react so plt tx tend to be ineffective unless large doses fiven to overpower antibodies
How is Post transfusion purpura treated
Large doses of platelets
Intravenous Ig
Steroids
Plasma exchange to remove Ab
What is TTP
TTP is caused by consumption of platelets in thrombotic lesions
Platelet transfusions are not indicated here
What is ITP?
Consumption of platelets caused by auto-Abs with broad specificities
Chronic vs acute
Talk about drug induced thrombocytopenia
Immune complexes bind to platelets which undergo RE removal a la drug induced haemolytic anaemia
Transfuse with platelets only in case of major haemorrhage
What is heparin induced thrombocytopoenia?
Thrombocytopenia caused by antibodies to heparin-platelet factor 4 complexes
These complexes bind to these platelet receptors resulting in activation
Thus thrombotic complications
Dont give these platelets as they will become activated
What is chronic ITP
Seen in adults
Insidious onset
X2 risk in women
Associated with HIV, malignancy and other auto-immune conditions
Treated with IVIG, steroids and splenectomy (same as AIHA)
What is acute ITP
Usually in children post viral infection
Steroids not used in children
If older then 6months splenectomy
What is DITP?
Drug induced Thrombocytopenia
Caused by drugs such as quinine (malaria), sulpha drugs, colloid gold (RA and Alzheimers)
Type I = non immune - self resolves
Type II = immune, serious, leads to thrombi in 30% of HIT cases and 0.9% of heparin patients
How is DITP treated?
Low molecular weight Heparin should be used
Or alternatively hirudin (from snake venom) an anti-thrombin drug
Risks and Hazards of plts
Febrile and allergic reactions
Overload -> as usually given in a major bleed
Disease transmission particularly bacteria - staph, strep, serattia
Transfusion of rbc alloantibodies
Sensitisation to rbc antigens
TAGVHD
Possible alternative in thrombopoietin
TRALI -> not a thing anymore since using male only donors
PTP, ABO incompatibliity etc
Why is platelet refractoriness often an annoyance?
Plts are often the last thing to increase but you cannot send a patient home with low plts
Talk about pathogen inactivation in platelets
We dont do this in ireland but we BacT all of ours
Psoralens show promise e.g. S59
Amotosalen-intercept technology is most common
Comment on the need for pathogen inactivation of platelets
Frequency as high as 1 in 1,000
Ranges from mild fever to septicaemia to death
Talk about psoralens
S59
A group of planar compounds
Form adducts with DNA and RNA and when exposed to UV light binding is irreversible, blocking nucleic acid replication
Virucidal and bactericidal properties
Talk about thrombopoietin
Produced primarily in the liver
Considerable homology to EPO
Increases with low plts, stimulates plt production by acting on pluripotent stem cells
Two main recombinant products available: rTPO and rMGDF
Why were TPOs taken off the market
Due to thrombosis formation
-> same as EPO
Give some other examples of platelet products
Frozen platelets in DMSO
Cold stored platelets @4 degrees for up to 2 weeks - being used in the USA in trauma settings - 7 day platelet incubation then 7 days warm - would prevent bacterial growth
Anti-fibrinolytic agents such as aprotinin and tranexamic acid
DDAVP
Lyophilised or freeze-dried platelets - blended into a powder but still seem to work
Microspheres of human albumin coated with fibrinogen
Thermbospheres - artificial platelets - much easier than red cells to make artifically - due to problems with Hb
