The partial thromboplastin time (PTT) test is a component of the diagnostic toolkit for evaluating blood coagulation disorders. Abnormalities in blood clotting can lead to significant health risks, including excessive bleeding or blood clot formation. Elevated PTT levels warrant careful investigation as they may indicate underlying conditions affecting the body's ability to form blood clots properly.
[signup]
Understanding the PTT Blood Test
The PTT test measures the time it takes for blood to clot (25).
What Is the Clotting Cascade?
The clotting cascade is a series of steps the body goes through to stop bleeding when you get a cut or injury. Imagine it like a line of dominoes that need to fall in a specific order to create a blood clot that plugs up the injury and prevents more bleeding. (6)
There are two main pathways in this process: the intrinsic pathway and the extrinsic pathway. Both pathways eventually meet at a common point, leading to the final steps that form the clot.
The intrinsic pathway starts entirely inside the blood vessels. It is triggered when the blood is exposed to certain surfaces inside the vessels, like the damaged inner lining of a blood vessel. This exposure activates proteins in the blood called clotting factors. These factors work like dominoes, activating one after another in a sequence until they reach a factor called factor X . Once factor X is activated, it moves into the common pathway. (6)
The extrinsic pathway begins outside the blood vessels when an injury damages the tissue surrounding a blood vessel. When this happens, the damaged tissue releases a substance called tissue factor. Tissue factor combines with a clotting factor VII. This combination activates factor X, which, like in the intrinsic pathway, leads to the common pathway. (6)
The clotting process enters the common pathway once the intrinsic or extrinsic pathway activates factor X. In this stage, factor X helps convert prothrombin (another protein in your blood) into thrombin. Thrombin then converts fibrinogen, a protein, into long strands of fibrin. Fibrin strands weave together to form a stable blood clot that seals the wound. (6)
What Is the PTT Test?
This test is one of several that help doctors evaluate the ability of a patient's blood to form a clot. It is used to measure the activity of the intrinsic and common pathways of the coagulation cascade. Specifically, the PTT assesses the function of clotting factors I, II, V, VIII, IX, X, XI, and XII. (3, 27)
A healthcare provider will order a PTT test in various clinical scenarios:
- Preoperative assessment to ensure patients have a normal coagulation profile before surgery
- Monitoring anticoagulant therapy, particularly unfractionated heparin
- Diagnosing and managing clotting disorders
- Evaluating unexplained bleeding and bruising
Normal PTT values can vary depending on the laboratory and the testing method. However, typical reference ranges for PTT are generally between 25 and 35 seconds. An abnormal result is one that falls outside this range, signifying potential blood clotting abnormalities (i.e., your blood is clotting too quickly or slowly). (24)
What Does a High PTT Mean?
A high PTT result indicates a prolonged time for blood to clot, meaning the clotting process is taking longer than expected. Clinically, a PTT value exceeding the upper limit of the normal range (over 35 seconds) is considered elevated. The degree of elevation can provide clues regarding the severity and potential cause of the coagulation abnormality. (24)
Symptoms associated with elevated PTT levels often depend on the underlying cause. Patients may experience:
- Unexplained bruising
- Prolonged bleeding from cuts
- Frequent nosebleeds
- Heavy menstrual periods (menorrhagia)
- Blood in the urine or stool
- Joint pain and swelling (4, 16)
In more severe cases, spontaneous internal bleeding, such as intracranial bleeding, might occur. Signs and symptoms of bleeding into the brain that warrant emergent medical attention include:
- Severe headache
- Vomiting
- Double vision
- Sudden muscle weakness
- Facial drooping
- Impaired ability to speak
- Confusion
What Causes High PTT?
The following medical conditions are associated with an elevated PTT time (14, 28):
- Hemophilia A and B: These are inherited genetic disorders that lead to excessive bleeding. A deficiency in factor VIII causes Hemophilia A, while a factor IX deficiency causes hemophilia B. Both are more common in males than females. (15)
- Von Willebrand Disease (VWD): This is another inherited blood clotting disorder caused by a deficiency or dysfunction of a protein called von Willebrand factor. It primarily affects platelet function, but VWD can prolong PTT in cases where factor VIII levels are concurrently reduced. (29)
- Vitamin K Deficiency: Vitamin K is a fat-soluble vitamin that plays a role in synthesizing blood clotting proteins. Vitamin K deficiency can result in deficiency of factors II, VII, IX, X, and proteins C and S, leading to prolonged bleeding time.
- Liver Disease: Because the liver makes blood proteins, consequences of advanced liver disease include reduced synthesis of clotting factors and excessive bleeding.
- Disseminated Intravascular Coagulation (DIC): This severe condition involves widespread activation of clotting pathways, eventually leading to the depletion of clotting factors and a prolonged PTT (5).
- Autoimmune Disease: Autoimmune diseases can prolong PTT by producing antibodies that interfere with the normal clotting process. For example, in conditions like lupus, the body may produce lupus anticoagulant antibodies that target and inhibit certain clotting factors, leading to a slower clotting process and a prolonged PTT.
Patients with blood clotting disorders that cause hypercoagulability (increased tendency to form blood clots) are given anticoagulant medications to prevent blood clot formation. If patients taking these medications, such as heparin, have an elevated PTT time, it indicates their medication dose is too high (24).
Lab Tests and Additional Testing
The PTT test is a straightforward blood test. No special preparation is usually required, although patients may be advised to avoid certain medications and supplements that could affect the test results, including antihistamines, aspirin, and vitamin C (14). During the procedure, a blood sample is drawn from a vein, usually in the arm, and sent to the lab for analysis. (24)
PTT vs. aPTT
The activated partial thromboplastin time (aPTT) is a variation of the PTT test that uses an activator to initiate the clotting process, leading to a more sensitive and shorter test time. aPTT is often preferred in clinical practice for its sensitivity to various coagulation abnormalities and its use in monitoring heparin therapy. Despite these differences, both PTT and aPTT tests serve similar diagnostic purposes and are often used interchangeably. (34)
Prothrombin Time (PT)
Prothrombin time (PT) differs from PTT because it measures how long it takes blood to clot by specifically evaluating the extrinsic and common pathways of the clotting cascade. It can help diagnose deficiencies in clotting factors I, II, V, VII, and X. PT is also commonly used to monitor patients on blood-thinning medications like warfarin. The results may be standardized using the international normalized ratio (INR) for consistent comparison across different labs. (33)
Thrombin Time (TT)
The thrombin time (TT) test measures the time it takes for a clot to form after thrombin is added to plasma. This test evaluates the final step of the coagulation cascade, where fibrinogen is converted into fibrin. TT is particularly useful in identifying deficiencies or abnormalities in fibrinogen and monitoring therapy with thrombin-inhibiting medications like heparin. (1)
Fibrinogen
Fibrinogen is a blood protein made by the liver that is converted into fibrin during the clotting process. Fibrin forms long, sticky strands that weave together to create a stable mesh, which acts like a bandaid to seal wounds and stop bleeding.
Factor Assays
Abnormal PTT results are typically followed up with a coagulation factor assay, a set of tests that measure the activity of specific clotting factors within the blood. These assays are particularly useful in diagnosing specific bleeding disorders, such as hemophilia A or B.
How to Treat High PTT
The treatment of elevated PTT depends on the underlying cause:
- Patients with hemophilia may require replacement therapy with clotting factor concentrates to normalize PTT levels and prevent bleeding.
- In cases of vitamin K deficiency, replenishing vitamin K stores through oral or subcutaneous supplementation can restore normal clotting function.
- Immunosuppressive therapies may be necessary for those with lupus anticoagulant antibodies to reduce the production of antibodies interfering with coagulation.
- In patients receiving heparin, your doctor will modify your dose and continue to monitor PTT to ensure it is within a target therapeutic range.
The nutritional needs of patients with blood clotting disorders can differ based on the specific cause of the disorder and any medications they may be taking. For instance, elevated PTT caused by vitamin K deficiency can in part be remedied by eating vitamin K-rich foods, such as leafy green vegetables.
Patients on anticoagulant therapy should consult their doctors before implementing major dietary modifications, as this may interfere with treatment efficacy. For example, vitamin K can make warfarin less effective.
Patients with elevated PTT should undergo regular follow-up testing to monitor their coagulation status and assess the effectiveness of any treatments initiated. The frequency of follow-up will depend on the severity of the PTT elevation, the underlying cause, and the patient's clinical status. Ongoing communication between the patient and healthcare provider is important for managing the condition effectively and preventing complications.
[signup]
Key Takeaways
- Elevated PTT levels indicate potential coagulation abnormalities and should be carefully evaluated to determine the underlying cause. From inherited disorders like hemophilia to acquired conditions such as liver disease, the causes of high PTT are diverse and require targeted interventions.
- Through appropriate testing, including the PTT, aPTT, TT, and factor assays, clinicians can identify the precise cause of the abnormality and tailor treatment accordingly.
- With proper management, including medication, dietary adjustments, and regular follow-up, patients with elevated PTT can reduce their risk of bleeding complications.
%201.svg){kind=logo}
