What laboratory tests are affected by Heparin (unfractionated heparin)?

From the Guidelines

Heparin significantly affects several laboratory tests, primarily those related to coagulation, including activated partial thromboplastin time (aPTT), activated clotting time (ACT), thrombin time (TT), and can cause falsely elevated D-dimer levels, as well as affecting antithrombin III assays, factor assays, and potentially causing pseudothrombocytopenia due to platelet clumping in collection tubes, as noted in studies such as 1 and 1. The most recent and highest quality study, 1, highlights the impact of heparin on coagulation tests, emphasizing the importance of considering these effects when interpreting laboratory results for patients on heparin therapy. Key points to consider include:

• Heparin's effect on aPTT, which is used to monitor unfractionated heparin therapy
• The potential for heparin to interfere with thrombin time (TT) and cause falsely elevated D-dimer levels
• The impact of heparin on antithrombin III assays and factor assays, particularly factors VIII and IX
• The possibility of pseudothrombocytopenia due to platelet clumping in collection tubes
• The importance of collecting blood samples just before the next heparin dose to minimize the impact on test results, as suggested by 1. It is crucial for healthcare providers to be aware of these potential interferences when interpreting laboratory results for patients on heparin therapy, to ensure accurate diagnosis and treatment, as emphasized by the need for careful monitoring and dose adjustment based on the results of coagulation tests, such as aPTT, as discussed in 1.

From the FDA Drug Label

Adjust the dosage of Heparin Sodium Injection according to the patient's coagulation test results. Dosage is considered adequate when the activated partial thromboplastin time (aPTT) is 1.5 to 2 times normal or when the whole blood clotting time is elevated approximately 2.5 to 3 times the control value. When initiating treatment with Heparin Sodium Injection by continuous intravenous infusion, determine the coagulation status (aPTT, INR, platelet count) at baseline and continue to follow aPTT approximately every 4 hours and then at appropriate intervals thereafter Periodically monitor platelet counts, hematocrit, and occult blood in stool during the entire course of heparin therapy, regardless of the route of administration.

The laboratory tests affected by Heparin (unfractionated heparin) are:

• Activated partial thromboplastin time (aPTT)
• International Normalized Ratio (INR)
• Platelet count
• Hematocrit
• Occult blood in stool
• Whole blood clotting time
• Aspartate aminotransferase (AST)
• Alanine aminotransferase (ALT) 2 2 2

From the Research

Laboratory Tests Affected by Heparin (Unfractionated Heparin)

• The anticoagulant effect of heparin is usually monitored by the activated partial thromboplastin time (APTT) 3, a test that is sensitive to the inhibitory effects of heparin on thrombin, factor Xa, and factor IXa.
• The APTT response can be affected by the type of clot detection system, the contact activator, and the phospholipid composition of the reagent 3.
• The prothrombin time (PT) test can also be affected by heparin, although it is more commonly used to monitor oral anticoagulant therapy 3.
• The thrombin time (TT) test can also be used to monitor heparin therapy, although it is less commonly used than APTT 4.
• The anti-Xa assay is another test that can be used to monitor heparin therapy, and it has been shown to be more accurate than APTT in some cases 5, 6.
• The activated clotting time (ACT) is also used to assess the anticoagulant effect of heparin, particularly in patients undergoing coronary angioplasty 7.

Factors Affecting Laboratory Test Results

• The type of reagent used can affect the results of laboratory tests, such as APTT and anti-Xa assays 4.
• The use of different instrumentation can also affect the results of laboratory tests, such as APTT and anti-Xa assays 4.
• The presence of other medications or medical conditions can affect the results of laboratory tests, such as APTT and anti-Xa assays 5, 6.
• The timing of laboratory tests can also affect the results, with some tests being more accurate at certain times than others 5, 6.

Comparison of Laboratory Tests

• The APTT and anti-Xa assays have been compared in several studies, with some showing that the anti-Xa assay is more accurate than APTT 5, 6.
• The ACT has also been compared to APTT, with some showing a strong correlation between the two tests 7.
• However, the results of laboratory tests can be discordant, with some patients having high APTT values relative to anti-Xa values 5, 6.