Laboratory Haematology as an Aid to Clinical Decision Making

Question 1

What is haematology?

Answer 1

Study of blood, blood-forming organs and blood diseases

Question 2

What is blood?

Answer 2

Specialised fluid (technically a tissue or organ) composed of cells suspended in a liquid called plasma
Only moving organ in body

Question 3

Why study blood?

Answer 3

All organs (exception is cornea) have a vascular supply

Question 4

How is blood tested?

Answer 4

Blood sample is in tube with anti-coagulant to prevent clotting, by trapping calcium that is involved with the coagulation cascade (tubes contain calcium chelators)

Placed in machine and, eventually, sample is retrieved:
rbcs settle at the bottom (fixed elements of blood)
Plasma sits on top
Two layers are separated by a buffy coat (wbcs and platelets)

Question 5

Components of blood?

Answer 5

Formed elements (~45%) - rbcs, wbcs and platelets
Two types of wbcs:
Granulocytes - have granules and include neutrophils, eosinophils and basophils
Agranulocytes - have no granules and are mononucleated like lymphocytes and monocytes

Plasma (~55%) - which is ~90% water and also includes proteins (mostly albumin, immunoglobins, clotting factors), nutrients and salts

Question 6

How are results generated?

Answer 6

Important to remember that measurements of formed elements of blood and plasma are done together, not separately

Question 7

How to interpret laboratory results?

Answer 7

In context of:
Normal range for that age, sex, hospital, etc; is the result lower or higher?
Physiology
Balance between production and destruction/loss, e.g: if trauma results in blood loss, can lead to loss of more blood cells than the bone marrow can produce

Question 8

Function of different types of cells and site of production?

Answer 8

rbcs - transport O2
wbcs - fight infection
Platelets - prevent bleeding

Majority produced in bone marrow

Question 9

Structure of red cells?

Answer 9

Do not have a nucleus; have Hb instead (iron-containing protein with a haem group)
At centre of haem group is iron atom
Lack of iron leads to reduction in Hb and iron-deficient rbcs are smaller
Stroage iron is reflected by a protein called FERRITIN measured in serum (if ferritin is low, iron is low)

Question 10

What is anaemia and how is it detected?

Answer 10

Reduced total rbc mass - not easy to measure in routine practice and Hb conc. is a surrogate marker

Question 11

Methods of classifying anaemia?

Answer 11

By pathophysiology (balance between destruction and production)
By morphological characteristics (features like size and contents of rbcs)

Question 12

Pathophysiological classification of anaemia?

Answer 12

Increased loss/destruction of rbcs via bleeding of haemolysis (breakdown of rbcs)

or

Decreased production due to maturation abnormalities from when cell had a nucleus (erythropoiesis is present but is INEFFECTIVE), or cytoplasmic defects (like impaired Haemoglobinisation, due to lack of iron), or nuclear defects (like impaired cell division)
Could be hypoproliferative - reduced amount of erythropoiesis

Question 13

Classification based on cell sizes?

Answer 13

Look at mean cellular volume:
MICROcytic - iron deficiency (e.g: chronic blood loss)
MACROcytic - vitamin B12/ folate deficiency can cause nuclear defects when cell still has a nucleus
NORMOcytic - in acute blood loss or in anaemia of CHRONIC disease (e.g: inflammation or infection) which is associated with decreased rbcs but normal sized

Question 14

What is involved with anaemia of chronic disease?

Answer 14

AKA inflammation
Cells of the reticuloendothelial system induce chages in:
Iron homeostasis
Proliferation of erythroid cells
Production of erythroprotein
Life span of rbcs

Question 15

What can associate with macrocytic cells without anaemia?

Answer 15

Alcohol excess
Liver disease
Hypothroidism

Question 16

Staining in blood film?

Answer 16

Combination of dyes:
One has affinity for nucleus - stains blue
One has affinity for cytoplasm - stains pink

Thus, can see rbcs (cells with no nucleus)

Question 17

Reasons for using blood films?

Answer 17

To confirm abnormalities identified by analysers
To identify additional abnormalities not detected by analysers:
Target cells in liver disease
Rbc fragmentation, reflecting mechanical damage to circulation rbcs
Polychromasia - reflecting an increase in immediate red cell precursors, called reticulocytes

Question 18

Summary of parameters to review in an anaemic patient?

Answer 18

Haemoglobin
rbc bount
Mean cellular volume (MCV)
Blood film report

Question 19

Causes of high Hb?

Answer 19

May not be true increase, e.g: dehydration can lead to increased blood conc.

Secondary causes - e.g: hypoxia driven or erythropoietin tumours

Question 20

When do neutrophils increase?

Answer 20

Main circulating phagocytes that increase with:
Stress - physiological/ pathological (acute infection, trauma, infarction and inflammation)
Steroids - cause de-margination (to centre of blood vessel, not to peripheries) so are more easily detected; this is NOT A TRUE CHANGE in neutrophils

Sometimes, will see “toxic”c changes in neutorphils on blood film comment (can occur in pneumonia)

Question 21

When do eosinophils increase?

Answer 21

Parasitic infections

Hypersensitivity/allergic reactions

Question 22

When do basophils increase?

Answer 22

Hypersensitivity reactions

Question 23

When do mononuclear wbcs increase?

Answer 23

Monocytes - CHRONIC infection, malignancy, autoimmune disorders
Lymphocytes (numerous types and functions) - in viral infections (like glandular fever: reactive or activated lymphocytes

Question 24

What is haemostasis?

Answer 24

Arrest of bleeding AND maintenance of vascular patency

Question 25

Requirements for haemostasis?

Answer 25

Permanent state of readiness
Prompt response
Localised response
Protection against unwanted thrombosis

Question 26

Steps in normal haemostatic system?

Answer 26

Formation of platelet plug - primary haemostasis
Formation of fibrin clot - secondary haemostasis
Fibrinolysis
Anticoagulant defences

Question 27

Describe platelets, their structure and function

Answer 27

Small anucleate discs with a mean life span of 7-10 days
Damage to endothelial vessel wall cells exposes collagen
Platelet ADHESION occurs at injured site
Then SECRETION of various chemicals from platelets, leading to AGGREGATION of platelets at site of injury

Question 28

Screening tests for primary haemostasis?

Answer 28

Platelet count

No simple screening tests for other components of primary haemostasis

Question 29

Causes of low platelet count?

Answer 29

Is it genuine first of all?
Could be:
Liver disease
Rapid consumption of platelets (acute blood loss, coagulopathy), drug-induced, autoimmune destruction

Question 30

Causes of high platelet count?

Answer 30

Generally, do not alter haemostasis (when between 450-1000 x 10 to power 9 platelets per litre)
Could be:
Acute blood loss
Iron deficiency
Inflammatory states
Malignancies

Question 31

How does fibrin clot formation occur?

Answer 31

AKA secondary haemostasis
Via an intrinsic or extrinsic pathway (come together to form factor X) that use calcium

Main points are:
Plasma = water + proteins (inc coagulation proteins/clotting factors)
Complex set of reactions involving different proteins/clotting factors is responsible for haemostasis
Clotting factors GENERALLY exist as an inactive protein, unless proteolytically activated
Activated clotting factors activate further coagulation reaction in presence of calcium

Reactions ultimately convert fibrinogen, soluble protein, to INSOLUBLE strands of fibrin, which, together with platelets, forms a stable thrombus

Question 32

Brief version of how fibrin clot forms?

Answer 32

Initiation - tissue factor VIIa comes together with V/Xa, along with VIII/IXa
Causes propagation of prothrombin to thrombin (amplification via thrombin forming more VIII/IXa)

Fibrinogen is converted to the thrombus

Question 33

Screening tests for fibrin clot formation?

Answer 33

Measure integrity of coagulation pathways:
Prothrombin time - measure TF/VIIa)
Activated partial prothrombin time - measures factor VIII and IX)

Question 34

Causes of failure to form fibrin clot?

Answer 34

Multiple clotting factor deficiencies (usually acquires, e.g: Disseminated Intravascular Coagulation, liver disease or anticoagulant drugs, like warfarin

or

Single clotting factor deficiency - usually hereditary, e.g: haemophilia

Question 35

What occurs in fibrinolysis?

Answer 35

Plasminogen converted to plasmin by tissue Plasminogen Activator (tPA)

Plasmin converts fibrin to FDPs (Fibrin degradation production), including D-dimers

Question 36

What are D-dimers?

Answer 36

Fibrin degradation products measured in haematology lab

If increased, suggests an increase in fibrinolysis (following increased fibrin deposition, e.g: post-surgery, heart failure, malignancy and abnormal thrombus)

Question 37

Use of D-dimer measurements?

Answer 37

D-dimer measurement can be combined with clinical scores to determine need for further investigation in patients with suspected thrombosis

Always use D-dimers with clinical score

Question 38

What is plasma viscosity?

Answer 38

Index of changes in plasma proteins (reactant fibrinogen and some globulins)

Changes in plasma viscosity can reflect systemic inflammation and, less commonly, haematological malignancies producing an abnormal protein