In a medical context, blood thickness refers less to physical viscosity and more to the blood’s ability to clot, known as coagulation. Coagulation must maintain a delicate balance: stopping bleeding after injury while preventing unwarranted clots from forming inside blood vessels. If blood clots too easily, it risks conditions like stroke or deep vein thrombosis. Conversely, if blood does not clot easily enough, it can lead to excessive bleeding, even from minor injuries. Monitoring this function is important for preventing these two extremes.
Measuring Blood Coagulation
Clinicians assess clotting tendency using specialized blood tests rather than measuring the physical viscosity of the blood. Coagulation tests measure the time it takes for a clot to form within a sample. The two main tests used are the Prothrombin Time (PT) and the Activated Partial Thromboplastin Time (aPTT).
The PT measures clotting proteins in the extrinsic and common pathways of the coagulation cascade. The aPTT test assesses clotting factors involved in the intrinsic and common pathways. Both tests report the result as the number of seconds required for the blood sample to clot after specific reagents are added.
Because the reagents used for the PT test vary between laboratories, the International Normalized Ratio (INR) was developed. The INR mathematically converts the PT result to a standardized value, allowing consistent comparison across different facilities. For patients taking specific types of anticoagulant medication, the INR is the most frequently monitored value.
Understanding Normal Reference Ranges
For a healthy individual not taking anticoagulant medication, the normal range for the International Normalized Ratio (INR) is between 0.8 and 1.2. A value within this range indicates the blood is clotting at an expected rate. The Prothrombin Time (PT) for a healthy person falls between 11 and 13.5 seconds.
The normal Activated Partial Thromboplastin Time (aPTT) for an unmedicated adult is between 25 and 35 seconds. Results higher than these reference ranges mean the blood is taking longer to clot, suggesting a higher risk of bleeding, or “thinner” blood. Conversely, a result below the normal range suggests the blood is clotting faster, indicating hypercoagulability or “thicker” blood.
Conditions That Affect Blood Thickness
Increased Bleeding Risk (Hypocoagulability)
Numerous health issues and external factors can cause the blood to clot too slowly, leading to a high INR or prolonged PT/aPTT. Since the liver produces many coagulation proteins, advanced liver disease is a common cause of hypocoagulability. A deficiency in Vitamin K, which is necessary for the synthesis of specific clotting factors, can also prolong clotting time.
Inherited conditions, such as deficiencies in specific clotting factors, increase the risk of excessive bleeding. Anticoagulant medications, often called blood thinners, are another significant cause of intentionally elevated INR and prolonged clotting times, as they are prescribed to slow the clotting process.
Increased Clotting Risk (Hypercoagulability)
When blood clots too readily, it indicates a hypercoagulable state, increasing the likelihood of deep vein thrombosis or pulmonary embolism. This condition can stem from inherited genetic mutations that disrupt the natural checks and balances of the coagulation system. For example, the Factor V Leiden mutation makes a clotting factor resistant to the protein that normally turns it off, leading to unchecked clot formation.
Acquired risk factors are more common. Prolonged periods of physical inactivity, such as extended bed rest or long-distance travel, increase the risk of clot formation by slowing blood flow. Other factors contributing to hypercoagulability include:
- Chronic conditions like cancer and certain autoimmune disorders.
- Obesity.
- Smoking.
- The use of estrogen-containing medications, such as some oral contraceptives.
Clinical Approaches to Management
Managing abnormal blood thickness requires careful adjustment to restore the proper balance between clotting and bleeding. For patients on warfarin, this involves regular INR monitoring to ensure the level is within the target therapeutic range, often between 2.0 and 3.0. If the INR is too high, the dose may be reduced or a Vitamin K supplement prescribed to reverse the thinning effect.
If a patient’s blood is too thick, the primary intervention is prescribing anticoagulant medications to reduce the risk of future clots. Management also focuses on treating underlying medical conditions, such as addressing inflammation. Lifestyle changes are commonly recommended to mitigate clot risk, including increasing physical activity, maintaining hydration, and quitting smoking.