Haematopoietics Flashcards
what is a bleeding disorder?
an abnormal condition which allows blood to escape from injured vessels or interferes with haemostasis following injury
in which species are bleeding disorders more common?
dogs > cats
what are the 2 stages of haemostasis?
primary and secondary haemostasis
what occurs in primary haemostasis?
reflex constriction of the blood vessel and formation of a platelet plug
what occurs in secondary haemostasis?
stabilisation of the platelet plug by fibrin, which results from activation of the clotting cascade
do primary and secondary haemostasis occur at different times?
no - in life, both are triggered simultaneously
why are vessels prone to trauma?
they are fragile and thin to allow transfer in and out
how does the vessel endothelium react to vessel injury?
secretes activating factors and proteins in response to a gap having formed
what occurs as a result of secretion of activating factors by the endothelium?
activates platelets, which migrate to the site of injury
how does vessel diameter respond to injury?
vasconstriction to slow blood flow to the area
how do platelets arrange locally at the site of injury?
during activation, receptors on the inside of the platelets are flipped onto the surface to make the platelets ‘stickier’ - clump at the site of injury
how is von Willebrands factor involved in primary haemostasis?
sticks to the activated proteins on the activated platelet surfaces and increases their stickiness even more
what is the first clot formed during primary haemostasis called?
primary haemostatic plug
what is the primary haemostatic plug?
the first clot formed during primary haemostasis
what process makes the primary haemostatic platelet plug more likely to persist?
vasoconstriction - helps in stopping the flow of blood down the vessel, helps protect the plug and makes it more likely that the clot will stick
what is von Willebrands disease?
deficiency of von Willebrands factor
what breed commonly suffers from von Willebrands disease?
Dobermanns
what is the most commonly inherited haemostatic disorder in dogs?
von Willebrands disease
what is the result of von Willebrands disease?
platelet adhesion/clumping impaired
what is the result of secondary haemostasis?
formation of fibrin
what is the purpose of fibrin?
scaffolds the platelets and stabilises/anchors the primary haemostatic platelet plug
why does secondary haemostasis occur?
isn’t essential for small injuries
bigger injuries require both primary and secondary haemostasis for the vessel deficit to heal
what are clotting factors?
protease enzymes which catalyse a cascade of reactions that break down proteins to make fibrin from fibrinogen
what are the 2 arms of the fibrin cascade?
intrinsic and extrinsic pathways
what happens to the intrinsic and extrinsic pathways of the fibrin cascade?
come together into a common pathway for the later stage where prothrombin is converted to thrombin
how is prothrombin converted to thrombin?
via the extrinsic and intrinsic pathways
what is the role of thrombin in the fibrin cascade?
catalyses conversion of fibrinogen into fibrin
what does fibrin do?
stabilises the primary haemostatic platelet plug
why can liver disease lead to coagulopathies?
all clotting factors are made in the liver
why does rodenticide poisoning lead to coagulopathy?
they work by binding vitamin K - required to recycle vitamin K clotting factors
why does rodenticide poisoning show a delayed clotting dysfunction?
takes a couple days for the working clotting factors in the blood to get used up before needing to be recycled by vitamin K
which arm of the clotting cascade is affected by vitamin K deficiency?
extrinsic arm
what type of vitamin is vitamin K?
fat-soluble
which cats may develop clotting disorders?
anorexic cats, cats with fat digestion issues due to blocked bile ducts (stone or pancreatitis)
(vit K is fat-soluble)
what are the possible defects of primary haemostasis?
decreased platelet number - IMTP, infectious diseases
decreased platelet function - end -stage CKD, multiple myeloma
vasculitis (adder bite)
what are the possible defects of secondary haemostasis?
quantitative disorders - decreased amount of clotting factors
qualitative disorders - decreased function of clotting factors
when do inherited bleeding disorders usually present?
<6 months
how can sex affect inherited bleeding disorders?
males are affected by sex-linked (X chromosome) coagulation disorders e..g haemophilia
why is it important to find out about previous trauma/surgery/toxins/drug exposure?
any previous trauma/surgery will usually have resulted in bleeding complications
recent toxin exposure may help to point towards the cause
what are primary haemostatic diseases typically characterised by?
multiple minor bleeds
prolonged bleeding
what are secondary haemostatic diseases typically characterised by?
single large bleeds
rebleeding
what are the clinical signs of primary haemostatic diseases?
petechiae and ecchymotic (larger) haemorrhages
often multiple sites of bleeding
prolonged bleeding from cuts/venepuncture
surface bleeding common - mms, skin, eyes
what are the clinical signs of secondary haemostatic diseases?
haematomas common
often localised site of bleeding
delayed bleeding or rebleeding from cuts
venepuncture usually uncomplicated
deep and cavity bleeds common - joints and abdominal/thoracic cavities
what are the considerations for blood sampling with suspected bleeding disorders?
collect before starting any therapy
atraumatic venepuncture to avoid excessive activation of haemostasis and local consumption of platelets
proper handling and collection of samples
what tests can be used for primary haemostasis testing?
platelet count - in-house estimation
buccal mucosal bleeding time
vWF testing (sodium citrate)
what is the BMBT?
screening test for platelet defects (number/function) and vessel wall defects
what is the normal BMBT?
<3 mins in cats
<3.5 mins in dogs
why is the BMBT more likely to be used to assess platelet function?
vessel wall disorders are rare
when might the BMBT test for platelet function?
BMBT can be prolonged in cases where the platelet count is normal but platelet function is impaired, e.g. von Willebrands disease
should a BMBT be carried out if thrombocytopaenia has been identified?
no - already know it will be prolonged
can a BMBT be carried out on a conscious patient?
can usually be done conscious in dogs
cat will require heavy sedation/GA
what position should the patient be in for a BMBT?
lateral recumbency - upper lip folded and held in place with gauze bandage tied around thr muzzle/head
how tight should the lip bandage be for a BMBT?
should mildly obstruct venous return
how is a BMBT carried out?
a pair of small standardised incisions are made in the buccal mucosa with a spring loaded bleeding time device
blood is blotted away using filter paper, WITHOUT disturbing the incision sites
time taken for cessation of bleeding is recorded
what can we use for identification of quantitative platelet disorders?
platelet counts and platelet count estimation
what blood tube should be used for samples intended for platelet counts?
EDTA
how can we perform an estimation of platelet count in-house?
examination of a stained (diff-quik) blood smear made from fresh EDTA anticoagulated blood
how is an estimation of platelet count performed?
low power - check for any platelet clumps (will influence count)
oil immersion - count platelets per field, 10 fields total
what is represented by each platelet per high power field?
represents approx 20 x 10^9/l platelets in the circulation
how many platelets in each high power field is considered normal?
11-25
what does activated clotting time evaluate?
intrinsic and common pathways
how does the tube for ACT measurement work?
contain diatomaceous earth which activates the intrinsic pathway
why are the first few drops of blood discarded when testing ACT?
in case of endothelial activation factors interfering with the test results
give a brief overview of testing for ACT
first few drops of blood discarded
2mls blood collected into test tube, gently invert to mix
leave undisturbed for 40 mins
tube inverted every 10 seconds and the time taken for complete clot formation is recorded
what is evaluated by the activated partial thromboplastin time (APPT)?
evaluates the intrinsic and common pathways
what is the advantage of APPT over ACT?
it is more sensitive than the ACT
what is evaluated by the prothrombin time?
extrinsic and common pathways
PT is very sensitive to vitamin K deficiency and rodenticide toxicity
how are the APPT and PT performed?
blood sampled into sodium citrate tubes and filled exactly to marked line
samples must be evaluated immediately, or double spun down to recover the citrate plasma and frozen until transported
does APTT and PT always have to be sent away for?
can be done on some in-house analysers
thromboelastography starting oy be used in referral hospitals (haemostatic profile)
why is anaemia more common in cats?
cats mask illness and disease - diagnosed later
lifespan of feline RBC (70 days) is shorter than dogs (110 days)
total RBC mass lower than in dogs
feline haemoglobin has low affinity for oxygen - anaemia is tolerated well
what does it mean if feline haemoglobin has a low affinity for oxygen?
readily give up oxygen to tissues
what are the clinical signs of anaemia in cats?
pale (or yellow) mms - most important sign
lethargy, weakness
hyperdynamic pulses
tachycardia (>200bpm)
heart murmur
tachypnoea
enlarged lymph nodes and spleen
PICA
why do haemic murmurs occur with anaemia?
due to reduced viscosity of blood
why does anaemia lead to widespread lympadenopathy and splenomegaly
increased removal of RBCs
what additional signs may be seen with anaemia?
signs associated with the underlying cause e.g. infection with FeLV/FIV/FIP
how might PICA present in anaemic cats?
owners have reported licking of concrete, eats cat litter or soil
what is the first priority in investigation of anaemia?
haematological investigation - PCV
how can a patient have a normal PCV but still have anaemia?
due to acute haemorrhage - volume of blood lost will reduce overall number of RBCs in direct proportion with other blood components
why are reference ranges for PCV different for dogs and cats?
red blood cell density is lower in the cat than the dog
what PCV ratio characterises chronic anaemia?
lower proportion of RBCs to plasma
what PCV ratio characterises volume overload?
more plasma proportionally to RBCs, PCV normal
what haematological features suggest the anaemia is regenerative?
reticulocytes >50x10^9/l
anisocytosis
polychromasia
MCV increased
MCHC decreased (cells less concentrated, with increased reticulocytes)
what haematological features suggest the anaemia is non-regenerative?
reticulocytes <50 x 10^9/l
normocytic
normochromic
MCV normal
MCHC normal
what is anisocytosis?
variation in RBC size
what is polychromasia?
variation in RBC density of colour (haemoglobin)
what does hypochromic mean?
pale (less haemoglobin per RBC)
what does it mean if a RBC is nucleated?
earlier release from the bone marrow
what does normocytic mean?
normal size
what does microcytic mean?
small RBC
what does macrocytic mean?
large RBC
why is it normal to see some punctate reticulocytes in the cat?
because it takes longer (10d) for their RBCs to mature
what type of reticulocytes can develop in the cat?
aggregate and punctate
what type of reticulocyte do dogs have?
aggregate
what type of reticulocytes represent active bone marrow regeneration in cats?
aggregate - rarely seen in normal cats, unlike punctate
what does the presence of aggregate reticulocytes mean in cats?
active bone marrow regeneration - regenerative anaemia
what stain is required for reticulocyte counts?
new methylene blue
what is the best way to establish whether an anaemia is regenerative or not??
performing an absolute reticulocyte count
how do we determine the absolute reticulocyte count?
absolute reticulocyte count (x10^9/l) =
observed % reticulocytes / RBC count (x10^12/l) x 10
why is it important to determine whether an anaemia is regenerative or not?
important for differential diagnoses - regenerative anaemias arise only due to blood loss or haemolysis
what are the main confounding factors which make it more difficult to categorise anaemia?
duration of anaemia
concurrent disease
why is duration of anaemia a confounding factor in categorising anaemia?
regenerative - takes 2-5 days for retics to be released from bone marrow and appear in circulation, in this time will appear to be non-regenerative
how can anaemia be affected by speed of blood loss?
slow bleeding e.g. chronic GI blood loss
eventually leads to iron deficiency anaemia, which impairs erythropoiesis - started off as regenerative but, with time, becomes non- or poorly regenerative anaemia
are cats commonly affected by iron deficiency anaemia?
relatively rare except in kittens with chronic flea infestation
how is concurrent disease a confounding factor in categorising anaemia?
many chronic diseases (inc FeLV), both infectious and inflammatory, can reduce bone marrow response to anaemia, making regenerative causes of anaemia appear non-regenerative
what are the main causes of haemorrhage causing regenerative anaemia?
trauma
coagulopathies
chronic blood loss from flea infestations, infected tumours, GI tract
what type of anaemia results from chronic blood loss from flea infestations/infected tumours/GI tract?
non-regenerative microcytic hypochromic iron deficiency anaemia
(due to depleted iron reserves)
what are the different potential causes of regenerative anaemia due to haemolysis?
infectious - FeLV and FIA
immune-mediated - drugs, neoplasia, FeLV
Heinz body anaemia
severe hypophosphataemia (refeeding syndrome)
incompatible blood transfusions
neonatal isoerythrolysis
inherited defects (rare)
which findings may suggest an underlying haemolytic anaemia?
pre-hepatic jaundice (uncommon)
haemoglobinuria (uncommon)
lymphadenopathy/splenomegaly as a result on increased RBC destruction
is immune mediated haemolytic anaemia common in cats?
less common as a primary condition (unlike in dogs) - seen secondary to drugs (e.g. methimazole), some cancers, FIA
what do Heinz bodies represent?
irreversibly denatured oxidised haemoglobin
are Heinz bodies a normal finding in cats?
<5% RBCs with Heinz bodies can be normal
why might Heinz body anaemia occur in a cat?
oxidative damage caused by:
paracetamol toxicity
onion toxicity
lymphoma
DKA
how do incompatible blood transfusions cause haemolysis of RBCs?
antibodies in donor/recipient plasma attack naturally occurring antigens on the RBCs
how does neonatal isoerythrolysis occur?
occurs when kittens nurse and drink milk high in anti-A antibodies
A kittens born to B queens
how can neonatal isoerythrolysis be avoided?
screen queens and toms - only breed B toms with B queens
which breeds are more likely to be affected by neonatal isoerythrolysis?
brachy breeds - persians, birmans, british shorthairs
what causes feline infectious anaemia?
Mycoplasma haemofelis
how is Mycoplasma haemofelis trasmitted?
fleas
what are the signs of feline infectious anaemia?
pallor
lethargy
anorexia
weight loss
pyrexia
dehydration
jaundice if severe acute disease
how can feline infectious anaemia be diganosed?
PCR of blood - 0.5ml EDTA required
how is feline infectious anaemia treated?
doxycycline antibiotic followed by food/syringed water - risk of oesophagitis if stays in oesophagus
what type of anaemia is a cat most likely to have?
most common is non-regenerative
what is a non-regenerative anaemia?
where the bone marrow fails to respond adequately to the anaemia, and does not produce adequate new RBCs
why might a non-regenerative anaemia occur?
can arise due to disorders of the bone marrow itself OR suppression of normal bone marrow response due to systemic disease
what is required to diagnose non-regenerative anaemia when severe?
bone marrow sampling
where is bone marrow sampling carried out in cats?
humerus or femur (under GA)
what conditions might cause a secondary non-regenerative anaemia?
neoplasia
FeLV/FIV/FIP
bacterial infections
CKD
chronic inflammation
how does chronic inflammation induce anaemia?
may induce a mild-moderate non-regenerative anaemia, partly due to marrow suppressive effects of circulating toxins, but also due to sequestration of iron reserves
how severely can anaemia due to inflammatory disease develop?
very common but rarely severe
how does CKD induce anaemia?
variety of mechanisms:
reduced RBC lifespan
blood loss
impaired iron utilisation
EPO deficiency
where/when is erythropoietin produced?
produced by the kidneys in response to hypoxia
what is the effect of erythropoietin?
stimulates RBC production in the bone marrow
when might a blood transfusion be required in an anaemic cat?
often critical on presentation - may be necessary as a life-saving measure
when might a blood transfusion be used as an adjunct treatment?
cases with FIA and non-regenerative anaemias, or whilst awaiting response to specific therapy or results of diagnostic investigations
what is the decision around blood transfusion based on?
clinical grounds, rather than a particular PCV cut-off
generally if PCV >10% transfusion will be unnecessary in chronically anaemic cats
why is transfusion unnecessary in chronically anaemic cats with PCV >10%?
due to compensatory mechanisms the cat develops with chronic anaemia - acute patients will be affected much more severely than those with gradual development
in which cats might erythropoietin therapy be useful?
those with anaemia secondary to CKD
what are the disadvantages of using erythropoietin to treat anaemia?
not all patients respond and there are a number of side effects
not licensed - must be used under cascade
how might we reduce side effects of erythropoietin treatment?
use of newer recombinant human erythropoietin
which treatment for anaemia is available in a primary care setting?
anabolic steroids e.g. laurabolin
why is anaemia often treated empirically?
in significant proportion of cases of marrow failure, no underlying cause can be determined
how do anabolic steroids help treat anaemia?
may stimulate erythroid precursors in bone marrow, via an effect on EPO activation
little convincing evidence of any convincing effect
when might prednisolone therapy be used for anaemia?
considered as treatment on the basis that an immune-mediated mechanism has been proposed in some cases of marrow failure
