Generality
Prothrombin is a glycoprotein that participates in the blood coagulation process.
In case of lesion of a blood vessel, conversion of prothrombin (factor II) to thrombin (factor IIa) occurs.
This triggers a chain reaction that leads to the formation of a blood clot .
Prothrombin time (PT = Prothrombin time ) is a laboratory analysis, which quantifies the time necessary for the formation of a clot:
- PT values higher than the reference ones indicate that the blood takes longer than normal to coagulate;
- Lower values indicate that the blood coagulates more quickly than normal.
For this reason, the prothrombin time assessment is prescribed even when a person is taking anticoagulant drugs.
What's this
Prothrombin, also known as coagulation factor II, is a plasma glycoprotein synthesized by the liver.
The lesion of a blood vessel determines the rapid conversion of prothrombin (factor II) into thrombin (factor IIa), which in turn converts fibrinogen into an insoluble fibrin polymer; a fiber weave is thus produced which participates in the formation of the clot.
Why do you measure
By measuring the prothrombin time it is possible to evaluate:
- How effective is coagulation;
- How long it takes for the blood to clot.
Moreover, this parameter allows to verify the functioning of the blood thinning drug (anticoagulant) and its effectiveness in preventing the formation of clots.
The test result can be presented in two different ways:
- Prothrombin time in seconds : indicates the time needed for the blood to coagulate. The reference values may vary slightly from laboratory to laboratory, but are generally between 11 and 13 seconds.
- Prothrombin time as ratio (INR) : the international normalized ratio (INR, international normalized ratio) allows to eliminate the variability of the results obtained in the different laboratories. This parameter is usually used for people taking anticoagulant drugs, in which case it should be between 2.0 and 3.0. In the absence of particular problems, on the other hand, values between 0.9 and 1.3 are expected.
When is prothrombin time measured?
The doctor may prescribe prothrombin time analysis to monitor the efficacy of anticoagulant drugs (eg, warfarin) or as a support in diagnosing haemostatic disorders and liver diseases .
Prothrombin time and INR
The time of protombina (PT, from the English Prothrombin Time) is an analysis of the blood able to quantify the time necessary for the formation of a fibrin clot. For this purpose, specific substances are added to the sample, such as citrate, calcium and tissue thromboplastin.
Under normal conditions, prothrombin time varies from 11 to 13 seconds, depending on the analytical methods adopted. In most cases, however, the prothrombin time is expressed by an index called INR (International Normalized Ratio), which takes into account the sensitivity of the thromboplastin reagent used. In this way, the doctor can evaluate the results accurately, even when they come from laboratories that use different methods of determination.
Prothrombin time, expressed as INR, is very important for monitoring patients receiving oral anticoagulants .
Under normal conditions, the optimal value of INR is between 0.9 and 1.3. However, based on the patient's characteristics and therapeutic needs, the doctor can establish optimal values of higher INRs; for example, in the case of atrial fibrillation or in the prevention of venous thrombosis, the ideal INR is between 2 and 3, while in patients with mechanical valve prostheses the adequate INR is a little higher, between 2.5 and 3.5.
Prothrombin time and partial thromboplastin time
Prothrombin time is useful for evaluating five of the twelve coagulation factors (I-fibrinogen -, II - prothrombin, V - proaccelerin, VII - proconvertin and X - prothrombinase).
All these factors are synthesized by the liver and three of these (II, VII and X) are activated by vitamin K-dependent enzymes.
Oral anticoagulants, such as coumadin - warfarin, are vitamin K antagonists and as such inhibit the activation of the aforementioned coagulation factors. By acting in this way, these drugs "thin the blood", preventing clots from forming inside the circulatory stream.
The activity of the other coagulation factors (intrinsic and common way) is detected by a laboratory blood test called partial thromboplastin time (PTT) . To get a more complete picture of blood coagulation activity, the two tests are often performed simultaneously, especially in the presence of the following indications:
Symptoms of an excess clotting disorder :
- Thrombosis;
or at fault :
- Nose blood loss;
- Bleeding gums;
- Hypermenorrhea (particularly abundant menstrual flow);
- Blood in stool or urine;
- Loss of vision;
- Chronic anemia;
- Ecchymoses and hematomas.
Normal values
For those who do not take drugs, normal values are included in the following ranges:
- Prothrombin time (PT): 11-13 seconds;
- INR: 0.9-1.3.
For those taking anticoagulants, the ideal INR value is between 2 and 3.
Prothrombin time High - Causes
Prothrombin time is extended by:
- Use of anticoagulant drugs, such as coumadin (warfarin), sintrom (acenocumarol) or more rarely heparin;
- Absence, poor activity, reduced synthesis or excessive consumption (as in disseminated intravascular coagulation) of coagulation factors I, II, V, VII and X ; the test is particularly sensitive to the deficiencies of factors VII and X;
- Vitamin K deficiency due to reduced dietary intake or poor intestinal absorption (also due to a hypolipidic diet or due to the presence of a biliary obstruction);
- Liver diseases, such as cirrhosis, hepatitis or liver failure.
Consequences
In the presence of a delay in the normal blood coagulation process (high INR), small bleeding may occur. Slight blood loss can occur, for example, in the gingival (teeth washing), nasal or anal (due to the presence of hemorrhoids); typical is the appearance of small ecchymoses (bruises) on the skin following trauma even of modest entity.
In addition, if there are clotting delays, anemia, fainting, excessive menstruation, blood in the urine and joint problems may occur. In many cases, during collection, the staff has difficulty detecting the vein.
Prothrombin time Low - Causes
A low prothrombin time means that the blood coagulates faster.
A decrease in the PT may depend on:
- Taking barbiturates, oral contraceptives or hormone replacement therapy.
- Increased intake of vitamin K (through supplements or foods such as liver, broccoli, chickpeas or cabbage, green tea or soy-based products);
- Autoimmmuni diseases (lupus, rheumatoid arthritis etc.);
- Nephrotic syndrome;
- Increase in antithrombin.
Consequences
Possible complications associated with low prothrombin time include:
- Thrombus formation;
- Defects in anticoagulation.
How to measure it
A simple blood test is sufficient to measure prothrombin time.
Factors influencing prothrombin time
Prothrombin time can be influenced by several variables.
In subjects treated with anticoagulants, the most important factor is the dietary intake of vitamin K; for this reason, it is essential that these individuals follow a regular and balanced diet, which brings as many constant amounts of vitamin K every day (attention also to vitamin supplements.
To learn more: Diet for coumadin
Barbiturates, oral contraceptives and hormone replacement therapies, can shorten the time of the prothrombin.
Severe diarrhea, prolonged vomiting and any other cause responsible for ispissatio sanguinis (dehydration) may increase INR values; furthermore, diarrhea can lead to a prolongation of prothrombin time due to the relative lack of vitamin K induced by the lack of intestinal absorption.
Preparation
Although it is not strictly necessary, the test taken in the morning, fasting for at least 8 hours, is optimal for performing the prothrombin time examination and avoids any interference.
If you are following anticoagulant therapies, the withdrawal should be done before the daily dose.
Interpretation of Results
Prothrombin time is required above all to detect blood clotting abnormalities. However, the examination also allows to evaluate liver function, since prothrombin is produced by the liver.
High Values
The high prothrombin time indicates that the blood tends to coagulate more slowly, so the patient is exposed to the risk of bleeding. If so, the anticoagulant dose may need to be decreased.
A high prothrombin time could be an indication of liver problems, such as cirrhosis, hepatitis or liver failure.
Values higher than normal can be caused by many drugs, especially anticoagulants, antibiotics and anti-inflammatories. The same is true for vitamin K deficiency due to reduced food intake or poor intestinal absorption.
An increase in prothrombin time may depend, finally, on coagulation defects or dysfunctions (deficiency or alteration) of one of the factors involved in the process.
Low values
A low prothrombin time value means that coagulation is too rapid and therefore denotes a risk of clot formation; in this case, it may be necessary to increase the anticoagulant dose.
A decrease in prothrombin time may depend on the use of barbiturates, oral contraceptives or hormone replacement treatments.
