Blood is the body fluid most frequently used for analysis. Blood sampling is undertaken to assess body processes and disorders and includes haematology, biochemistry and arterial blood gas analysis (Chernecky and Berger [32], Keogh [114], Pagana and Pagana [177], WHO [257]).
Indications
Indications for blood sampling include:
defining baseline results
establishing a diagnosis
establishing a prognosis
confirming or screening for disease
ruling out a clinical problem
monitoring disease
regulating therapy or treatment.
Methods of blood sampling
The methods of blood sampling vary depending on the blood sample required and include venepuncture, sampling via vascular access devices and arterial sampling. The samples obtained via these means will have different properties; for example, venous blood will be low in oxygen in comparison to an arterial sample (Pagana and Pagana [177]).
It is important that the correct blood tube is used for each test. 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 guidelines. Correct ‘order of draw’ should be followed to avoid transferring additive from one tube to another when filling (see Table 13.4) (Garza and Becan‐McBride [65], Pagana and Pagana [177]).
Methods of investigation
Numerous blood tests are available. Blood samples are sent to various departments within the laboratory, such as haematology, biochemistry or microbiology. Brief outlines of some routine tests are given below. Refer to specialist reference texts for more detail.
Haematology
The full blood count is the most commonly requested blood test (Higgins [84]). It involves monitoring the levels of red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). Variations to normal values can indicate anaemia, infection or thrombocytopenia (Table 13.2).
Table 13.2 Haematology
Test
Reference range
Functions and additional information
Red blood cells
Men: 4.5–6.5 × 1012/L
Women: 3.9–5.6 × 1012/L
The main function of the RBCs is the transport of oxygen and carbon dioxide using haemoglobin (
Pagana and Pagana [177]).
Haemoglobin
Men: 135–175 g/L
Women: 115–155 g/L
Haemoglobin is a protein pigment found within the RBCs that carries oxygen.
Anaemia (deficiency in the number of RBCs or haemoglobin 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.
White blood cells
Men: 3.7–9.5 × 109/L
Women: 3.9–11.1 × 109/L
The function of the WBCs 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 × 109/L 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 or 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 [15]).
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 [15]).
INR, international normalized ratio; RBC, red blood cell; WBC, white blood cell.
Group and save (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 blood type: A, B, O or Rh (Rhesus). All staff should receive formal documented training in blood transfusion practice (Robinson et al. [210]) (see Chapter c12: Respiratory care, CPR and blood transfusion).
Biochemistry
Urea and electrolytes are the most common biochemistry tests requested (Table 13.3).
Table 13.3 Biochemistry
Test
Reference range
Functions and 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 due to fluid loss from diarrhoea, excessive sweating, increased urinary output or a poor oral intake of fluid. An increased salt intake may also cause an elevation.
Hyponatraemia (serum sodium <135 mmol/L) may be indicated in fluid retention (oedema).
Potassium
3.5–5.2 mmol/L
Potassium plays a major role in nerve conduction, muscle function, acid‐base balance and osmotic pressure. It has a direct effect on cardiac muscle, influencing cardiac output by helping to control the rate and force of each contraction.
The most common cause of hyperkalaemia (serum potassium >5.2 mmol/L) is chronic renal failure. The kidneys are unable to excrete potassium. The level may be elevated due to an increased intake of potassium supplements during treatment. Tissue cell destruction caused by trauma or cytotoxic therapy may cause a release of potassium from the cells and an elevation in the potassium plasma level. It may also be observed in untreated diabetic ketoacidosis.
Urgent treatment is required as hyperkalaemia may lead to changes in cardiac muscle contraction and cause subsequent cardiac arrest.
The main cause of hypokalaemia (serum potassium <3.5 mmol/L) is the loss of potassium via the kidneys during treatment with thiazide diuretics. Excessive or chronic diarrhoea may also cause a decreased potassium level.
Urea
2.5–6.5 mmol/L
Urea is a waste product of metabolism that is transported to the kidneys and excreted as urine. Elevated levels of urea may indicate poor kidney function.
Creatinine
55–105 μmol/L
Creatinine is a waste product of metabolism that is transported to the kidneys and excreted as urine. Elevated levels of creatinine may indicate poor kidney function.
Calcium
2.2–2.6 mmol/L
Most of the calcium in the body is stored in the bone but ionized calcium, which circulates in the blood plasma, plays an important role in the transmission of nerve impulses and the functioning of cardiac and skeletal muscle. It is also vital for blood coagulation.
High calcium levels (hypercalcaemia >2.6 mmol/L) can be due to hyperthyroidism, hyperparathyroidism or malignancy. Elevation in calcium levels may cause cardiac arrhythmia, potentially leading to cardiac arrest (
Chernecky and Berger [32], Pagana and Pagana [177]).
Tumour cells can cause excessive production of a protein called parathormone‐related polypeptide (PTHrP), which causes a loss of calcium from the bone and an increase in blood calcium levels. This is a major reason for hypercalcaemia in cancer patients (
Higgins [84]).
Hypocalcaemia (<2.20 mmol/L) is often associated with vitamin D deficiency due to inadequate intake or increased loss due to gastrointestinal disease. Mild hypocalcaemia may be symptomless but severe hypocalcaemia may cause increased neuromuscular excitability and cardiac arrhythmias. It is also a common feature of chronic renal failure (
Higgins [84]).
C‐reactive protein (CRP)
<10 mg/L
Elevation in the CRP level can be a useful indication of bacterial infection. CRP is monitored after surgery and for patients who have a high risk of infection. CRP level can help clinicians to monitor the severity of inflammation and assist in the diagnosis of conditions such as systemic lupus erythematosus, ulcerative colitis and Crohn's disease (
Higgins [84]).
Albumin
35–50 g/L
Albumin is a protein found in blood plasma that assists in the transport of water‐soluble substances and the maintenance of blood plasma volume.
Bilirubin
(total) <17 μmol/L
Bilirubin is produced from the breakdown of haemoglobin; it is transported to the liver for excretion in bile. Elevated levels of bilirubin may cause jaundice.
Liver function tests
There are numerous tests that are used to assess liver function. These include alkaline phosphatase (AP), gamma‐glutamyl transpeptidase (GGT), aspartate aminotransferase (AST) and alanine aminotransferase (ALT).
Microbiology
Various types of sample may be sent to the microbiology laboratory for screening, for example microbiological drug assays. Blood samples sent to microbiology may require screening for hepatitis B, hepatitis C and HIV.
