Blood Disorders And Coagulation Pathology Notes

Tests For Bleeding Disorders

 

Tests For Bleeding Disorders Introduction

Clinical laboratory evaluation is an integral part of the diagnosis and management of patients with bleeding disorders. Laboratory tests for hemostatic disorders depend on the component of hemostasis that is involved.

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Most bleeding disorders are caused by one of three defects:

1. A defect in platelet number or function,
2. A defect in platelet-vessel wall interactions (i.e. an abnormality in the adhesive   interactions between platelets and the vessel wall) or
3. A defect or deficiency in the coagulation factor.

The diagnosis of bleeding disorders requires a battery of tests which include screening tests done in all patients with a history of bleeding, followed by specific tests to identify the exact nature of the disorder.

Laboratory tests for hemostatic disorders depending on the component involved:

Laboratory tests in bleeding disorders:

Tests For Bleeding Disorders

Bleeding Time (BT):

• Bleeding time is used as a screening test for disorders of platelet-vessel wall interactions.
• It measures the time required for bleeding to stop after a standardized superficial cut of the skin capillary bed.

Template Method for BT:

• The template is a disposable blade fit onto a holder made of plastic and is used for the test.
• The blade projects through the bottom so that the incision made through the slit is 9 mm long and 1 mm deep.

Principle of BT:

• A small skin cut of a standard size and depth is made and the oozing blood is wiped with a filter paper.
• The bleeding stops when the capillaries contract and the platelet plug seals the vessel.

The procedure of BT :

• The patient is made to sit on a chair with an armrest so that the forearm is steady and exposed.
• The skin of the forearm is cleaned with alcohol and allowed to dry.
• The sphygmomanometer CUF is tied to the upper arm and the cuff is inflated to 40 mm Hg.
• A site on the forearm is selected away from the superficial veins.
• Place the template 5 cm distal to the antecubital crease (on the volar surface).
• One cut is made using a smooth rapid movement with the template (in the simulate/surgical test, no pressure is applied to the device
• On pressing the trigger an incision of 5 mm length and 1 mm depth develops).
• Immediately after the incision, a stop watch is started.
• Another cut is made 1.5–2 cm away from the first one and another stop watch is started.
• The blood oozing from the sides of the cut is blotted with fitter paper (Whitman No.1), every 30 seconds until the bleeding stops.
• Do not touch the wound edges to avoid disturbing the clot as it is formed.
• The time at which bleeding stops is noted for both cuts.
• Place a butterfly adhesive bandage over the site of puncture to avoid scarring.
• The average of the two readings is the bleeding time for the patient.
• Normal Range: 2–9 minutes.

Uses of BT:

• The test is prone to problems of reproducibility, sensitivity, and specificity.
• Though it is one of the screening tests, it is not recommended as a routine preoperative screening test.
• This test evaluates the defects of primary hemostasis.
• In this, bleeding time measures the platelet-vessel wall interactions (integrity of capillary and platelet function).

Note: The bleeding time usually is not prolonged in patients with coagulation factor deficiencies.

Interpretation of BT:

Prolonged bleeding time is found in:

• Platelet disorders:
  • Quantitative: Thrombocytopenia. If the platelet count is below 50,000/ mm3, BT should not be performed as bleeding may be difficult to stop.
  • Qualitative: von Willebrand disease, Bernard-Soulier syndrome, Glanzmann’s thrombasthenia.
• Primary vascular disorders: Ehlers-Danlos syndrome.
• Platelet-vessel wall interactions: von Willebrand disease.
• Others: Afirinogenemia, severe hypofibrinogenemia, uremia, aspirin.

Coagulation Or Clotting Time (Lee-White Method)

It measures the time taken for the fresh blood to clot.

 Procedure of Coagulation:

• Draw 3 mL of venous blood with aseptic precautions.
• Label 3 test tubes as No. 1, 2, and 3 and keep them in a water bath at 37 °C.
• Deliver 1 mL of blood into each of the above 3 test tubes and start the stopwatch.
• After 3 minutes, take out tube No 1, and tilt it every 30 seconds till a clot develops.
• Note the time when the tube can be inverted completely.
• Next examine tube No 2 every 30 seconds, exactly the same way as tube No 1, till the clot forms, and note the time.
• Finally, invert the third test tube as above till blood clots. Stop the watch.
• Record the time from the moment blood is delivered into the test tube to the complete clotting in the third tube.
• The clotting time of the third tube is reported as the clotting time.

Normal: 4–11 minutes.

Disadvantages of Coagulation:

• Not a sensitive test: It is one of the oldest tests and is not sensitive as it fails to detect mild/ moderate procoagulant defects.
  • Hence, it is obsolete now and not recommended as a screening test.
  • PTT is a more sensitive test for assessment of the coagulation cascade.
  • Clotting time is prolonged only with severe deficiency of factor 8,  9 or fibrinogen (afirinogenemia) and in heparin therapy.
• Misleading: Normal value may be obtained in mild-to-moderately severe hemophilia A and B, and it does not exclude major factor deficiency.
• Other use: Following the clotting time, the tubes can be left in the water-bath and examined after one hour for clot retraction test.

Quick’s One Stage Prothrombin Time

Principle of Prothrombin time:

• Prothrombin time (PT) is the time taken by citrated plasma to clot after the addition of tissue thromboplastin and calcium.
• It tests extrinsic and common pathway of coagulation system.

Uses of Prothrombin time:

• Screening test to evaluate coagulation disorders:
  • It measures coagulation factor 1, 2, 5, 7 and 10.
  • Deficiency of any one of these factors leads to prolongation of PT.
  • It should be used along with PTT.
  • To monitor oral anticoagulant therapy.
  • To evaluate liver function: Liver disease can result in deficiency of the coagulation factors.
  • Hence, PT should be performed before a liver biopsy and prolonged PT is a contraindication for liver biopsy.
• Normal Range: 11–16 seconds

Interpretation: Prolonged PT is seen in:

• Liver disease.
• Administration of oral anticoagulants like Coumadin.
• Vitamin K deficiency
  • Obstructive jaundice.
  • Hemorrhagic disease of the newborn.
• Deficiency of factors 1, 2, 5, 7 and 10.
• Disseminated intravascular coagulation (DIC).

Activated Partial Thromboplastin Time (Partial Thromboplastin Time)

Principle of Activated partial thromboplastin time:

• Activated partial thromboplastin time (APTT) is the time taken for citrated plasma to clot in the presence of a surface activator (kaolin), phospholipid and calcium.
• Partial thromboplastin time (PTT) is a measure of the intrinsic and common coagulation pathways.

Uses Activated partial thromboplastin time:

• Best single screening test for coagulation disorders. This test is abnormal with deficiencies of I2, 5, 8, 9, 10, 11 and 12.
• For screening—hemophilia A and B.
• For detecting coagulation inhibitors.
• For monitoring anticoagulant therapy like heparin.
• Normal Range: 30–40 seconds.

Interpretation Activated partial thromboplastin time:

Common causes of a prolonged APTT are:

• Inherited coagulation disorders deficiency of factor 2, 5, 8, 9, 10, 11, 12 (for example, Hemophilia A, hemophilia B).
• von Willebrand disease.
• Disseminated intravascular coagulation.
• Liver disease.
• Heparin therapy.
• Vitamin K deficiency.
• Oral anticoagulant therapy.