TOPIC 7 Haematology and coagulation
Laboratory tests of coagulation 137
Tests: Prothrombin time (PT)/international normalized ratio (INR), activated partial thromboplastin time (APTT) and thrombin time (TT) 137
Common first tests
Test: Full blood count and peripheral blood smear (PBS)
Indications
• To show abnormalities in the production, life span, and destruction of blood cells and aid in the diagnosis of anaemia, polycythaemia, thrombocytosis, thrombocytopenia, leucopaenia and leucocytosis.
• As a preoperative baseline in cases with expected significant blood loss (see NICE guidelines at http://www.nice.org.uk).
How it is done
• Automated blood counters using either forward angle light scatter or impedance analysis to provide:
– Erythrocyte mean cell volume (MCV), haematocrit (Hct), red cell distribution width (RDW), a measure of cell size scatter
Interpretation
Table 7.1 Normal adult ranges for blood values
| Male | Female* | |
|---|---|---|
| Haemoglobin (g/dL) | 13–18.0 | 11.5–16.5 |
| RBCs (×1012/L) | 4.5–6.5 | 4.0–5.8 |
| Hct (%) | 0.40–0.52 | 0.37–0.47 |
| MCV (fL) | 84–96 | 84–96 |
| MCH (pg) | 27.0–32.0 | |
| MCHC (g/dL) | 27.0–32.0 | |
| Platelets (×109/L) | 150–400 | |
| WBCs (×109/L) | 4.0–11.0 | |
| Neutrophils (×109/L) | 2.0–7.5 | |
| Lymphocytes (×109/L) | 1.5–4.0 | |
| Monocytes (×109/L) | 0.2–0.8 | |
| Eosinophils (×109/L) | 0.04–0.4 | |
| Basophils (×109/L) | 0.0–0.1 | |
| Reticulocytes (% or 109/L) | 0.5–2.5 or 20–80 |
* In pregnancy the Hb may fall as low as 9 g/dL in the third trimester. RBCs, red blood cells; Hct, haematocrit; MCV, mean cell volume; MCH, mean cell haemoglobin; MCHC, mean corpuscular haemoglobin concentration; WBC, white blood cell.
Abnormalities
Polycythaemia: Increased haemoglobin/haematocrit
| True polycythaemia | Secondary polycythaemia | Apparent or spurious polycythaemia |
|---|---|---|
| Polycythaemia rubra vera, (PRV) | Inappropriate erythropoietin secretion in benign & malignant renal disorders and by some tumours | Secondary to cigarette smoking, obesity, excess alcohol or hypertension |
Thrombocytopenia: Platelet count <150 × 109/L
Table 7.4 Causes of thrombocytopenia
| Failure of production | Increased consumption | Pseudothrombocytopenia |
|---|---|---|
| Caused by clumping: can be excluded by examination of PBS and citrate sample (clumping due to EDTA) |
Thrombocytosis: Platelet count >450 × 109/L
Table 7.5 Causes of thrombocytosis
| Primary myeloproliferative disorder | Secondary reactive disorder |
|---|---|
Leucopenia: WBC <4.0 × 109/L
• It is uncommon for absolute leucopenia to be due to an isolated deficiency of any cell other than the neutrophil.
| Neutropenia | Lymphopenia | |
|---|---|---|
| Congenital causes | Acquired causes | |
Drugs – cytotoxic agents, anticonvulsants, thyroid inhibitors, antibiotics, clozapine, procainamide, hydroxychloroquine, penicillamine, NSAIDs | ||
Management principles
• The full blood count is often performed unnecessarily as a preoperative screening tool. Various studies have shown that this is a waste of resources although some would argue that it is indicated in all premenopausal women because of the higher incidence of anaemia.
• NICE guidelines are available indicating those patients who should have a full blood count performed, mainly those with significant comorbidity or undergoing major surgery.
Test: Group and screen/crossmatch
Indications
• To determine a patient’s ABO blood group, and to screen serum for the presence of antibodies to common red cell antigens.
How it is done
• ‘Group’ – determines which ABO and Rh antigens are present on patient’s red blood cells (RBCs). RBCs are incubated with commercially available antibodies (anti-A, anti-B), which react with antigens if present and cause agglutination. The patient’s serum is then incubated with A and B cells to determine the presence of anti-A and anti-B antibodies. (See Table 7.8)
• ‘Screen’ – utilizes the indirect Coomb’s test (indirect antiglobulin test). Serum is incubated with a wide range of RBCs that together exhibit a comprehensive range of surface antigens. If antibodies are present they will cause agglutination.
• Crossmatch – may be electronic or serological. An electronic crossmatch is used in uncomplicated cases with a negative antibody screen. A serological crossmatch requires RBCs from the donor unit to be incubated with the patient’s serum, which will cause agglutination if the unit is not compatible.
Interpretation
Physiological principles
• Blood type is determined by the presence or absence of inherited antigenic material on the surface of RBCs; the ABO blood group system and Rh blood group system are the two most likely to cause harmful immunologically mediated reactions if noncompatible blood is transfused.
• ABO system. The A and B antigens are produced from a common precursor, the H antigen. Either, both or none may be present. Anti-A or anti-B IgM antibodies are produced in the first years of life possibly by sensitization to environmental substances such as food, viruses and bacteria.
Management principles
• ABO compatibility. When considering blood group compatibility for transfusion it is the recipient antibodies and donor antigens that are important. For example, group AB individuals have A and B antigens on the surface of their RBCs; their blood serum does not contain any antibodies against either A or B antigen so they can receive blood from any group (in theory, AB being preferable). Blood group O individuals have neither antigen but both anti-A and anti-B antibodies therefore will cause agglutination of the donor RBCs if they are given anything other than group O blood (Table 7.9).
• Rhesus compatibility. An RhD-negative patient with no antibodies could only receive RhD-positive blood once, as this would lead to the formation of antibodies and potentially hazardous transfusion reactions if they were to receive it again. If an RhD-negative woman develops antibodies and becomes pregnant with an RhD-positive child, these antibodies can cross the placenta and cause haemolytic disease of the newborn. Therefore rhesus-positive blood or platelets should never be given to RhD-negative women of childbearing age or patients with rhesus D antibodies. Platelets can be given with anti-D cover in emergencies.
• Plasma compatibility. Donor-recipient compatibility is the opposite of blood compatibility. Type O plasma (fresh frozen plasma, FFP) can only be given to type O recipients; Type AB can be given to individuals of any blood group (Table 7.10).
• There are usually local policies in place for the routine ordering of preoperative G&S or crossmatched blood that take into account the planned surgery, estimated blood loss and need for transfusion.
• Guidelines are available for the transfusion of blood and blood products published by The Association of Anaesthetists of Great Britain and Ireland (AAGBI) and British Committee for Standards in Haematology (BCSH) and advice should be sought from a haematologist where necessary.
Table 7.10 Plasma compatibility
| Recipient blood type | Donor plasma must be: |
|---|---|
| AB | AB |
| A | A or AB |
| B | B or AB |
| O | O, A, B or AB |
Laboratory tests of coagulation
Tests: Prothrombin time (PT)/international normalized ratio (INR), activated partial thromboplastin time (APTT) and thrombin time (TT)
Indications
• Assessment of coagulopathy in disseminated intravascular coagulation (DIC) and massive blood transfusions.
How it is done
• PT/INR – thromboplastin and calcium added to PPP and time to fibrin formation measured by a photo-optical or electromechanical device; this equals the prothrombin time (PT). To standardize results the PT is compared to a reference value, which gives the INR.
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