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A Practical Guide to Laboratory Haemostasis

 

The Bleeding Time [BT]



Introduction

The Bleeding Time (BT) was introduced as a tool for predicting the risk of bleeding in relation to surgery. It is a test that is no longer widely performed in the UK due to the difficulties in standardisation and the wide intra- and inter-user variability.


Principles & Method

The Ivy method is the traditional method for carrying out this test. In the Ivy method, a blood pressure cuff is placed on the upper arm and inflated to 40 mmHg. A disposable lancet is used to make two separate cuts into the forearm usually 5-10cm apart in quick succession. A stopwatch is started immediately and every 30 seconds filter paper is used to draw off the blood. The time from when the incision is made until all bleeding has stopped is called the bleeding time. The filter paper should not touch the edge of the clot as this may disturb the formation of the platelet plug. The test is finished when bleeding has stopped completely.
An attempt to standardise the method [Template Method] involves the use of an automatic blade which makes a standard-sized incision [approximately 6mm in length x 1mm in depth] on the volar aspect of the forearm. Otherwise the technique is identical.

Historically measuring the Bleeding Time involved the use of the ear lobe - the so-called Duke Method. However, the ear lobe is highly vascular and is no longer used.

Scarring can occur at the site of the BT and patients should be warned of this. For these reasons, the BT is rarely performed in children.

Interpretation

The test is dependent upon an adequate number of functionally active platelets that can adhere to the endothelium to form aggregates. The test is abnormal i.e. prolonged in:

Bleeding Time Abnormalities:
Collagen disorders e.g. Ehlers Danlos syndrome
Thrombocytopaenia It is important to check the platelet count before performing a bleeding time.
A platelet count of <50 x 10/L is generally considered to prolong the BT.
Qualitative platelet disorders Inherited and acquired platelet disorder including the use of anti-platelet drugs such as aspirin and clopidogrel will prolong the BT. However, the BT cannot reliably predict the risk of peri-operative bleeding in patients taking these drugs.
Paraproteinaemias can also lead to defective platelet function and may, therefore, prolong the BT.
Other acquired disorders of platelet function such as seen in uraemia and the myelodysplastic syndromes (MDS) and myeloproliferative disorders (MPD) will also prolonged the BT.
Von Willebrand Disease (VWD) A deficiency of Von Willebrand Factor (VWF) may prolong the BT but not in all cases. The BT is no longer recommended as a test for the diagnosis of Von Willebrand Disease (VWD)- see UKHCDO guidelines on Diagnosis of VWD.
Severe anaemia In patients with anaemia, there is a change in the distribution of platelets and a decreased interaction of the platelets with the vascular endothelium resulting in a prolonged BT. Correction of the anaemia will improve the BT.
Hypofibrinogenaemia Fibrinogen is required for platelet-platelet interaction and the BT will, therefore, be prolonged in cases of hypofibrinogenaemia.

 

Reference Ranges

The reference range for this test is between 2-7 minutes. In cases in which the BT exceeds 20 minutes it is usual to stop at 20 minutes and report the BT as >20minutes.

What Test Next

 - In patients in whom the BT is prolonged - the causes outlined above should be considered. Appropriate investigations in addition to taking a history of drug exposure would include:
 - Full Blood Count including platelet numbers and morphology
 - Platelet function testing - platelets
 - Measurement of Von Willebrand Factor activity
 - Consider the possibility of a collagen vascular disorder

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.