BLEEDING TIME

When a sharp-pointed knife is used to pierce the tip of the finger or lobe of the ear, bleeding ordinarily lasts for 1 to 6 minutes. The time depends largely on the depth of the wound and the degree of hyperemia in the finger or ear lobe at the time of the test. Lack of any one of several of the clotting factors can prolong the bleeding time, but it is especially prolonged by lack of platelets.

CLOTTING TIME

Many methods have been devised for determining blood clotting times. The one most widely used is to collect blood in a chemically clean glass test tube and then to tip the tube back and forth about every 30 seconds until the blood has clotted. By this method, the normal clotting time is 6 to 10 minutes. Procedures using multiple test tubes have also been devised for determining clotting time more accurately.

Unfortunately, the clotting time varies widely, depending on the method used for measuring it, so it is no longer used in many clinics. Instead, measurements of the clot ting factors themselves are made, using sophisticated chemical procedures.

PROTHROMBIN TIME AND INTERNATIONAL NORMALIZED RATIO

 Prothrombin time gives an indication of the concentration of prothrombin in the blood. Figure 1 shows the relation of prothrombin concentration to prothrombin time. The method for determining prothrombin time is the following.

Fig1. Relation of prothrombin concentration in the blood to  prothrombin time. 

Blood removed from the patient is immediately oxalated so that none of the prothrombin can change into thrombin. Then, a large excess of calcium ion and tissue factor is quickly mixed with the oxalated blood. The excess calcium nullifies the effect of the oxalate, and the tissue factor activates the prothrombin-to-thrombin reaction by means of the extrinsic clotting pathway. The time required for coagulation to take place is known as the prothrombin time. The shortness of the time is determined mainly by prothrombin concentration. The normal prothrombin time is about 12 seconds. In each laboratory, a curve relating prothrombin concentration to prothrombin time, such as that shown in Figure 1, is drawn for the method used so that the prothrombin in the blood can be quantified.

The results obtained for prothrombin time may vary considerably even in the same individual if there are differences in activity of the tissue factor and the analytical system used to perform the test. Tissue factor is isolated from human tissues, such as placental tissue, and different batches may have different activity. The international normalized ratio (INR) was devised as a way to standardize measurements of prothrombin time. For each batch of tissue factor, the manufacturer assigns an international sensitivity index (ISI), which indicates the activity of the tissue factor with a standardized sample. The ISI usually varies between 1.0 and 2.0. The INR is the ratio of the person’s prothrombin time (PT) to a normal control sample raised to the power of the ISI:

The normal range for INR in a healthy person is 0.9 to 1.3. A high INR level (e.g., 4 or 5) indicates a high risk of bleeding, whereas a low INR (e.g., 0.5) suggests that there is a chance of having a clot. Patients undergoing warfarin therapy usually have an INR of 2.0 to 3.0.

Tests similar to that for prothrombin time and INR have been devised to determine the quantities of other blood clotting factors. In each of these tests, excesses of calcium ions and all the other factors besides the one being tested are added to oxalated blood all at once. Then the time required for coagulation is determined in the same manner as for prothrombin time. If the factor being tested is deficient, the coagulation time is prolonged. The time itself can then be used to quantitate the concentration of the factor.

اخر الاخبار

اخبار العتبة العباسية المقدسة

مؤسّسة الوافي تُهدي مكتبةَ مسجد الكوفة نسخةً من موسوعة فتوى الدفاع الكفائي

المجمع العلمي يقيم برنامج (سراج المنتظرين) في محافظة المثنى

قسم الشؤون الفكرية ينظم ندوة فكرية عن أبعاد نهضة الإمام الصادق (عليه السلام)

العتبة العباسية المقدسة تناقش تكثيف جهودها الإغاثية لدعم الشعبين الإيراني واللبناني

المزيد

الآخبار الصحية

دواء جديد يحمي القلب ويعزز علاج السرطان في آن واحد

سر غذائي بسيط لحماية القلب مع التقدم في العمر

علامات تشير إلى اختلال عمل الغدة الدرقية

السمنة واللقاحات.. دراسة تكشف تأثيراً سلبياً

المزيد

الآخبار التكنلوجية

البعوض في آيسلندا إشارة مبكرة على تحولات بيئية في القطب الشمالي

صاروخ يطلق قمرا اصطناعيا في مدار خاطئ

"أوبن إيه آي" تطلق نموذجا للعلوم الحيوية واكتشاف الأدوية

تفوق على البشر.. روبوت يحطم الرقم القياسي لسباق نصف ماراثون !

المزيد

مواضيع ذات صلة

Western Blot Immunoassays

Indirect Fluorescent Antibody Tests and Other Immunomicroscopic Methods

Enzyme-Linked Immunosorbent Assays

Laboratory Diagnosis of Thyrotoxicosis

Laboratory Findings of Beta-Thalassemia Syndromes

Complement Fixation Assays

Flocculation Tests

Particle Agglutination Tests

Direct Whole Pathogen Agglutination Assays

Separating IgM From IgG For Serologic Testing

Serodiagnosis of Infectious Diseases

TSH- Receptor Antibodies