Can Aspirin Affect Blood Tests? Understanding the Impacts

October 12, 2025 •

3 min read

According to the American Academy of Family Physicians, many medications, including common over-the-counter drugs like aspirin, can influence the outcome of blood tests. Understanding how can aspirin affect blood tests is crucial for both patients and healthcare providers to ensure the most accurate interpretation of results.

Quick Summary

Aspirin significantly influences blood tests, primarily affecting coagulation markers and potentially causing anemia from microbleeding with long-term use. It can also impact liver, kidney, and glucose measurements depending on dosage.

Key Points

Aspirin is an antiplatelet agent: It inhibits platelet aggregation, directly impacting coagulation tests and prolonging bleeding time.
Long-term use can cause anemia: Chronic, low-dose aspirin can lead to subtle gastrointestinal bleeding, resulting in lower hemoglobin and ferritin levels, especially in older adults.
High doses can affect liver function: High therapeutic doses of aspirin may elevate liver enzymes (ALT/AST), though this effect is typically reversible upon stopping the medication.
Kidney function can be influenced: In susceptible patients, high doses or even mini-doses can cause reversible changes to kidney function, impacting creatinine and BUN levels.
Specific glucose tests can be inaccurate: Some continuous glucose monitors and A1C tests can produce inaccurate results due to salicylate interference.
Full disclosure is crucial: Always inform your healthcare provider about all medications, including aspirin, before any blood test to ensure proper interpretation of results.

How Aspirin Affects Coagulation Tests

Aspirin is known for its antiplatelet activity, which affects blood coagulation tests. It inhibits the enzyme COX-1 in platelets, reducing the production of thromboxane A2 (TXA2). TXA2 promotes platelet clumping, so aspirin's action prevents platelets from aggregating.

This affects several tests:

Platelet Function Assays: These tests will show reduced platelet function in individuals taking aspirin.
Bleeding Time: Aspirin use can prolong bleeding time, though results can vary.
Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT): Low-dose aspirin generally does not significantly impact routine PT and aPTT tests. High doses of salicylates may affect coagulation factors, potentially altering results, but this is less common with low doses.

Aspirin's Impact on Hematology and Anemia

Long-term, low-dose aspirin use has been associated with changes in blood cell counts, particularly an increased risk of anemia in older adults. Aspirin can interfere with protective substances in the GI tract lining, potentially leading to subtle internal bleeding over time. Studies indicate older adults taking daily low-dose aspirin have a higher likelihood of developing anemia and slightly lower hemoglobin and ferritin levels compared to placebo groups. This often mild anemia can cause symptoms like fatigue. Monitoring hemoglobin levels in older patients on long-term aspirin is advisable.

Effects on Liver and Kidney Function Tests

Aspirin can influence liver and kidney function tests, depending on dosage and the individual's health. High-dose aspirin may elevate liver enzymes (ALT and AST), which usually return to normal after stopping the medication. Low-dose aspirin is generally safe for those with reduced kidney function, although even mini-doses can influence creatinine and uric acid handling in the elderly. For more details on Aspirin's effects on the liver, see {Link: NIH https://www.ncbi.nlm.nih.gov/books/NBK548900/}. Aspirin and other NSAIDs can alter kidney function, particularly in those with pre-existing conditions. High doses can cause a reversible decline in function, affecting creatinine and BUN levels.

Interference with Glucose and Other Specific Tests

Aspirin can also interfere with tests for blood glucose and other markers. Salicylates in aspirin can affect the accuracy of some CGMs, potentially causing falsely low readings. CGM users should review device information and consult their doctor if taking aspirin. Aspirin and some other medications may lead to inaccurate A1C results. Patients may be told to avoid over-the-counter medications like aspirin for at least 12 hours before a glucose tolerance test.

Specific Test Interference Comparison Table


Type of Test	Primary Effect of Aspirin	Notes and Clinical Considerations
Coagulation Assays	Inhibits platelet aggregation	Most significantly affects specialized platelet function assays and bleeding time. Routine PT/PTT typically unaffected by low doses.
Complete Blood Count	May cause anemia (low hemoglobin/ferritin) with long-term use	Occurs due to chronic GI microbleeding, especially in older adults. Monitor for fatigue and unexplained low hemoglobin.
Liver Function Tests (LFTs)	Elevated liver enzymes (ALT/AST) at high doses	Effects are usually reversible upon discontinuation. Higher risk in susceptible individuals, such as those with SLE or RA.
Kidney Function Tests	May cause reversible decline in renal function at high doses or in susceptible patients	Can affect creatinine, BUN, and uric acid levels. Mini-dose aspirin can cause changes, particularly in the elderly.
Glucose Tests	Potential for false readings, particularly with CGMs or high doses	Some CGMs may read falsely low. High-dose salicylates can lower blood glucose levels. Can affect A1C results.
Salicylate Level Test	Direct measurement of aspirin levels	This is the specific test for monitoring aspirin or salicylate toxicity.

The Importance of Full Disclosure to Your Doctor

Because aspirin and other medications can interfere with blood tests, it's vital to inform your healthcare provider about all medications, supplements, and vitamins you are taking before any lab work. This allows for accurate interpretation of results and helps avoid unnecessary follow-up tests. Always communicate any changes to your medication regimen. Specific pre-test instructions, including potentially temporarily stopping certain medications like aspirin, may be provided. Never stop a prescribed medication, especially low-dose aspirin for cardiovascular prevention, without consulting your doctor.

Conclusion: Navigating Blood Tests While on Aspirin

Aspirin can affect various blood tests, with effects depending on the test type and dosage. It primarily impacts platelet function and coagulation, but long-term use can also increase the risk of anemia due to chronic microbleeding. Additionally, aspirin may influence liver, kidney, and glucose test results. Open communication with your doctor about all medications is key to ensuring accurate interpretation of your test results and receiving appropriate medical care.

Frequently Asked Questions

You should not stop taking prescribed aspirin without consulting your doctor first. Your doctor will provide specific instructions based on the type of blood test and your medical history. For some tests, like a glucose tolerance test, you may be advised to avoid it for a short time.

Yes, long-term use of low-dose aspirin, especially in older adults, has been shown to increase the risk of anemia due to subtle, chronic bleeding. This can result in lower hemoglobin and ferritin levels, which are measured in a CBC.

Aspirin is an antiplatelet drug that inhibits platelet function, which can prolong bleeding time. Therefore, it directly affects specialized tests measuring platelet function and, to a lesser extent, routine coagulation tests like PT and aPTT.

Yes, salicylates from aspirin have been associated with potential interference on some CGM systems, which can lead to falsely low glucose readings. Users should always check their device’s specific information regarding drug interactions.

High doses of aspirin can cause elevated liver enzyme levels. This is a form of hepatotoxicity that is generally mild and resolves once the medication is stopped.

Aspirin can cause a reversible decline in kidney function, especially at high doses or in patients with pre-existing kidney issues or certain systemic conditions like lupus. It may also alter uric acid and creatinine levels.

Yes, a salicylate level test measures the amount of salicylates in the blood. This test is primarily used to diagnose salicylate poisoning or to monitor therapeutic levels in patients taking high-dose aspirin for inflammatory conditions.

No, aspirin does not significantly interfere with all blood tests. Its most pronounced effects are on coagulation, blood cell counts (hemoglobin/ferritin), and, at higher doses, liver and kidney function. Providing your doctor with a complete list of medications is the best practice.