Clotting time

The time required for a sample of blood to coagulate in vitro under standard conditions is called "clotting time".

Clotting is the formation of a jelly like substance over the valves of the blood vessels
resulting in stoppage of blood flow. Clotting is a natural defence mechanism to prevent
blood loss from the body. A clot is usually formed within 5 minutes after injury. Whenever
a blood vessel is cut there is a rush of platelets causes a cut or injury to be filled and
thus bleeding stops.clotting is initiated by two pathways i.e intrinsic pathway and extrinsic pathway. 

There are various methods for determining the clotting time, the most common being the capillary tube method. It is affected by calcium ion levels and many diseases. Normal value of clotting time is 5_8 minutes.other methods for measuring clotting time are slide method

"WHOLE BLOOD COAGULATION (CLOTTING) TIME (LEE-WHITE)"

Principle. The whole blood clotting time is a rough measure of all intrinsic clotting factors in the absence of tissue factors. Variations are wide and the test sensitivity is limited. Whole blood, when removed from the vascular system and exposed to a foreign surface, will form a solid clot. Within limits, the time required for the formation of the solid clot is a measure of the coagulation system.

Reagents and Materials.(1) Stop watch equipment for collection of blood. (2) 2 plastic syringes. (3) 3 clean, dry glass test tubes (10 x 75 mm). (4) Water/dry bath (at 37oC).

Procedure.(1) Label glass tubes #1, #2, and #3. (2) Collect at least 2 ml of blood in a plastic syringe. Discard this blood.(This prevents tissue thromboplastin from entering the blood sample.) Changesyringes.(3) Collect at least 5 ml of blood in the second plastic syringe.(4) Approximately 1 ml of blood is placed in each of the three glass test tubes. (#3 first, then #2, then #1) (5) The stopwatch is started as soon as the blood enters the first tube #3.(6) All tubes are placed into the 37ºC water bath.(7) Gently tilt tube #3 (45 angle) every 30 seconds, until the blood in it clots.(8) Thirty seconds after tube #3 clots, proceed with tube #2, tilting every 30seconds, until a clot is formed.(9) Thirty seconds after tube #2 is clotted, tube #1 is tilted until no flow ofblood is observed on tilting.(10) Record the time. The coagulation time is the time required for the bloodto clot in the last tube. (Tube #1)

Ref Range: This range should be between 5 to 10 minutes. Range of Values.(1)Normal values: 5 to 15 minutes.(2)Critical value:Less Than Greater Than Not applicable 15 minutes Special notes.(a) The following variables tend to decrease the clotting time: roughhandling of the blood specimen, presence of tissue fluids (traumatic venipuncture),frequent tilting of the tube, and unclean tubes.(b) The following variables tend to increase the clotting time: extremeincreases in temperature, variation in pH, and performance of the test at roomtemperature.(c) This test is of value primarily as it was used to follow heparin therapy. Its use as a screening procedure is limited due to its poor sensitivity.(d) The whole blood clotting time is affected mainly by defects in the intrinsic pathway factors and by defects in fibrin and fibrinogen. It is not sensitive to platelet abnormalities.(e) A prolonged clotting time immediately indicates impairedcoagulation, but a normal clotting time does not exclude many serious clotting defects.(f) One disadvantage of the whole blood clotting time is its relative lack of reproducibility.(g) This procedure has been replaced in most laboratories with the APTT, which is more reproducible and easily controlled.(h) The coagulation time is normal in thrombocytopenic purpura. This is explained by the fact that only a small number of thrombocytes need be present for normal coagulation to take place. Lee-White is not recommended for evaluating heparin therapy - Lacks reproducibility

^[1] Ref:Fundamental skills for the clinical laboratory professional - Jacquelyn Marshall 1993 U.S. ARMY MEDICAL DEPARTMENT CENTER AND SCHOOL FORT SAM HOUSTON, TEXAS HEMATOLOGY II 2000 ABC's of Interpretive Laboratory Data - S. Bakerman

References

1. ^ Fundamental skills for the clinical laboratory professional - Jacquelyn Marshall 1993 U.S. ARMY MEDICAL DEPARTMENT CENTER AND SCHOOL FORT SAM HOUSTON, TEXAS HEMATOLOGY II 2000 ABC's of Interpretive Laboratory Data - S. Bakerman