The prothrombin time (PT)

The test is performed by adding thromboplastin to the patient’s platelet-poor plasma, warming, and then adding calcium. The time to clot formation is recorded in seconds and the PT may be expressed as the ratio of the patient’s time to a normal control time. The thromboplastin used should have been calibrated to allow this result to be converted to the international normalised ratio (INR) – the ratio which would have been obtained if the international reference preparation for thromboplastin had been used in the test (see p. 80). The PT is essentially a measure of the efficiency of the extrinsic clotting system (factor VII) in addition to the functioning of factors V and X, prothrombin and fibrinogen.

Activated partial thromboplastin time (APTT)

This test is sometimes referred to as the partial thromboplastin time with kaolin (PTTK) or the kaolin cephalin clotting time (KCCT). Patient platelet-poor plasma is combined with contact factors (kaolin, phospholipid) and calcium and the time to clot formation recorded in seconds. The test measures the overall efficiency of the intrinsic pathway (i.e. factors VIII, IX, XI, XII) as well as the function of factors V, X, prothrombin and fibrinogen.

Quantitation of plasma fibrinogen

In most laboratories this has replaced the thrombin time as a first-line test. Several accurate methods are available for the quantitative assay of plasma fibrinogen. Fibrinogen is an acute phase reactant (see below) and is frequently elevated in sick patients. Causes of low levels include disseminated intravascular coagulation (DIC) and severe liver disease.

Common clinical causes of abnormal first-line coagulation tests are shown in Table 10.1. Second-line tests may be needed for more precise diagnosis. In mixing experiments (or correction tests) patient plasma is mixed with normal or factor-deficient plasma prior to repeating first-line tests. If a particular coagulation factor is thought to be lacking, a quantitative assay can then be performed. A circulating inhibitor of coagulation is suggested by failure of the coagulation abnormality to be corrected by the addition of normal plasma. Many routine tests are now automated. Most coagulation instruments rely on measurement of changes in optical density to detect clot formation.

ESR

In this simple and inexpensive test venous blood (in citrate anticoagulant) is drawn up into a vertical tube (Fig 10.1) and allowed to stand for 1 hour. The red cells settle out of suspension and the length of plasma cleared after the hour is measured. The normal values are less than 5 mm/hour in men and less than 7 mm/hour in women, although values of up to 15 mm/hour are not infrequent in those over 60 years old. The test mainly reflects fibrinogen levels but is also influenced by α₂-macroglobulin, immunoglobulins and albumin. These proteins buffer the electrostatic repellent forces on the red cell membrane and allow the cells to come together and form reversible aggregates or rouleaux which fall more quickly through the plasma. The ESR result is affected by the haemoglobin concentration with high values seen in anaemia and low values in polycythaemia. A fresh sample must be processed as the result also changes over time.

Plasma viscosity

This test also measures the acute phase response indirectly, the result correlating with fibrinogen and immunoglobulin levels. The plasma viscosity has some advantages over the ESR. The normal range is the same in males and females and the result is independent of haemoglobin concentration. The sample can be taken from the EDTA anticoagulated blood count bottle and the test does not need to be performed immediately. The normal range, which is temperature dependent, is detailed in Table 10.2. Plasma viscosity measurement has direct pathophysiological relevance in myeloma where very high values are seen in the hyperviscosity syndrome.

C-reactive protein (CRP)

This easily measured protein is elevated in most types of tissue injury. The CRP is usually increased within 6–8 hours of the insult. The normal range is up to 10 mg/L with levels of 10–40 mg/L in severe viral infections, levels of 40–300 mg/L in bacterial infections and levels over 300 mg/L in severe burns. CRP results are not influenced by anaemia.

Other possible measures of the acute phase reaction include quantitation of fibrinogen, haptoglobins, alpha-1-antitrypsin and anti-chymotrypsin. These all rise following tissue damage but some acute phase reactants (notably albumin and transferrin) actually fall.