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Blood tests

Evidence-based approaches

Rationale

Blood tests are routinely collected to:

confirm disease

monitor disease

regulate therapy/treatment.

It is important that the correct blood tube is used for each test. The blood tubes contain special additives relevant to the type of test required, usually indicated by the colour of the tube top. The practitioner should ensure that the correct tube is selected by referring to local hospital guidelines. Correct ‘order of draw’ should be followed to avoid transferring additive from one bottle to another when filling (Garza and Becan-McBride 2010).

Numerous blood tests are available. Blood samples are sent to various departments within the laboratory, such as haematology, biochemistry and microbiology. Brief outlines of some routine tests are given below. Please refer to specialist reference texts for more detail.

Haematology

The full blood count (FBC) is the most commonly requested blood test (Higgins 2007). The FBC involves monitoring the levels of red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). Variations to normal values can indicate anaemia, infection and thrombocytopenia (Table 11.3).

Table 11.3 Haematology

Test Reference range Functions/additional information

RBC Men 4.5–6.5 × 1012/L

Women 3.9–5.6 × 1012/L The main function of the RBC is the transport of oxygen and carbon dioxide

Hb Men 13.5–17.5 g/dL

Women 11.5–15.5 g/dL Haemoglobin (Hb) is a protein pigment found within the RBC which carries the oxygen

Anaemia (deficiency in the number of RBC or in the Hb content) may occur for many reasons. Changes to cell production, deficient dietary intake or blood loss may be relevant and need to be investigated further

WBC Men 3.7–9.5 × 109/L

Women 3.9–11.1 × 109/L The function of the WBC is defence against infection

There are different kinds of WBC: neutrophils, lymphocytes, monocytes, eosinophils and basophils

Leucopenia is a WBC count lower than 3.7 and is usually associated with the use of cytotoxic drugs

Leucocytosis (high levels of neutrophils and lymphocytes) occurs as the body’s normal response to infection and after surgery

Leukaemia involves an increased WBC count caused by changes in cell production in the bone marrow. The leukaemic cells enter the blood in increased numbers in an immature state

Platelets Men 150–400 × 109/L Women 150–400 × 109/L Clot formation occurs when platelets and the blood protein fibrin combine

A patient may be thrombocytopenic (low platelet count) due to drugs/poor production or have a raised count (thrombocytosis) with infection or autoimmune disease

Coagulation/ INR INR range 2–3 (in some cases a range of 3–4.5 is acceptable) Coagulation occurs to prevent excessive blood loss by the formation of a clot (thrombus). However, a clot that forms in an artery may block the vessel and cause an infarction or ischaemia which can be fatal (Blann 2007)

Aspirin, warfarin and heparin are three drugs used for the prevention and/or treatment of thrombosis

It is imperative that patients on warfarin therapy receive regular monitoring to ensure a balance of slowing the clot-forming process and maintaining the ability of the blood to clot (Blann 2007)

Blood transfusion

All patients who require a blood transfusion need to have their blood type confirmed. It is essential that correct patient identification and accurate labelling are maintained. The sample will be screened to determine the ABO and Rh (Rhesus) type. All staff should receive formal documented training in blood transfusion practice. (Refer to Chapter 8.)

Biochemistry

Urea and electrolytes are the most common biochemistry tests requested (Table 11.4).

Table 11.1 Biochemistry

Test Reference range Functions/additional information

Sodium 135–145 mmol/L The main function of sodium is to maintain extracellular volume (water stored outside the cells), acid/base balance and the transmitting of nerve impulses

Hypernatraemia (serum sodium >145 mmol/L) may be an indication of dehydration