Can antibiotics mess with bloodwork? Understanding medication and lab results

October 14, 2025 •

5 min read

Research has shown that antibacterials, including common cephalosporins and penicillins, are among the most likely drugs to interfere with laboratory tests. This confirms that yes, antibiotics can mess with bloodwork, leading to inaccurate or misleading results that can complicate diagnosis and treatment.

Quick Summary

Antibiotics can interfere with blood tests by altering lab values for white blood cells, liver function, kidney function, and glucose, or by causing false positives in drug screenings. These changes can result from the drug's biological effects, assay interference, or the underlying infection itself, making interpretation challenging.

Key Points

Infection vs. Drug Effect: An underlying infection can alter bloodwork (e.g., higher WBC count), and successful antibiotic treatment will cause these values to normalize, which can be mistaken for a drug effect.
Assay Interference: Some antibiotics can directly interfere with the chemicals used in lab tests, leading to false readings for things like creatinine, glucose, or a positive Coombs test, without a real change in the body.
Organ Function: Certain antibiotics can cause genuine, temporary changes in organ function, such as elevating liver enzymes (LFTs) or affecting kidney function markers (KFTs), which appear as abnormal lab values.
False Positives: In rare cases, antibiotics can cause false-positive results on sensitive tests, including some urine drug screens for opiates (e.g., rifampin).
Critical Timing: For blood cultures, samples must be taken before antibiotics are started to ensure accurate results, as the medication can kill the bacteria and cause a false negative.
Good Communication is Key: Always inform your healthcare provider about all medications, including antibiotics, to help them correctly interpret your lab results and avoid potential misdiagnosis.

How Antibiotics Interfere with Blood Tests

Antibiotics, while vital for treating bacterial infections, can have a complex relationship with the body's chemistry. This can lead to interference with the sensitive assays used in laboratory testing. This interference is not always due to a true pathological change, but can sometimes be a direct interaction between the drug compound and the testing chemicals, causing an inaccurate reading. Other times, the antibiotic's effect on the body's natural processes, or the body's response to the infection it is fighting, can cause a genuine, but temporary, shift in lab values.

There are several mechanisms through which this interference can occur:

Direct Assay Interference: Some antibiotics physically or chemically react with the reagents used in lab tests. For instance, certain cephalosporins can cause false-positive results in urine glucose tests and the direct Coombs test, which detects immune-mediated red blood cell destruction. The antibiotic cefoxitin is also known to spuriously increase serum creatinine levels by interfering with the colorimetric assay used to measure it.
Biological and Metabolic Effects: Antibiotics can directly affect organ function, like the liver or kidneys, or alter metabolic processes. For example, some antibiotics can cause a rise in liver enzymes or disrupt glucose metabolism.
Immunological Reactions: In some cases, antibiotics can trigger an immune response that affects blood components. This is seen in drug-induced neutropenia, where certain antibiotics cause an abnormal drop in white blood cells. Penicillins can also lead to immune-mediated hemolytic anemia, which can cause a positive Coombs test.
Underlying Infection: It is crucial to remember that the infection being treated by the antibiotic is also a major factor influencing bloodwork. For instance, an infection naturally causes a rise in white blood cell (WBC) count, and as the infection resolves with antibiotic treatment, the WBC count will return to normal. This drop could be mistaken for an antibiotic side effect if the full context is not considered.

Specific Blood Tests and Antibiotic Effects

Complete Blood Count (CBC)

White Blood Cell (WBC) Count: A primary effect of many antibiotics, particularly penicillins and vancomycin, is the potential to cause neutropenia, a low neutrophil count. This is often a temporary condition that resolves after the medication is stopped. However, the healing process itself can also cause fluctuations in WBCs as the body's immune response subsides.
Platelet Count: Some antibiotics, including cephalosporins, penicillins, and sulfa drugs, have been associated with thrombocytopenia, or a low platelet count. In rare cases, this can lead to excessive bleeding. In contrast, some conditions treated by antibiotics can cause low platelets, and successful treatment can help raise them.

Liver Function Tests (LFTs)

Elevated Enzymes: A variety of antibiotics can cause elevated liver enzymes (AST, ALT, ALP) and bilirubin. Examples include amoxicillin-clavulanate, which is a common cause of drug-induced liver injury, macrolides like erythromycin and azithromycin, and fluoroquinolones such as ciprofloxacin. Patients on these medications may show signs of jaundice and need monitoring.

Kidney Function Tests (KFTs)

Increased Creatinine: Some cephalosporins (cefalotin, cefazolin) and other antibiotics like trimethoprim can elevate creatinine levels. This can be due to direct kidney toxicity or, in some cases, an interference with the lab's measurement, resulting in a false reading.

Other Tests and Interferences

Glucose Tests: Certain antibiotics like amoxicillin can cause falsely high glucose readings. Fluoroquinolones can also affect glucose levels, potentially leading to either high or low readings.
Coagulation Studies (PT/INR): Antibiotics can affect the effectiveness of blood thinners like warfarin, leading to an increased International Normalized Ratio (INR), which indicates blood is taking longer to clot.
Drug Screens: Certain antibiotics, though rare, can trigger false positives on urine drug screens. For example, rifampin, used for tuberculosis, can sometimes be mistaken for opiates.

Lab Interference vs. True Effect

Distinguishing between a true, medication-induced change and an artifact of the laboratory test is critical for proper clinical interpretation. The context of the patient's illness and treatment is key. For instance, a temporary rise in WBC count is expected during an infection. The decline in this count as antibiotics work is a sign of recovery, not necessarily an adverse drug effect. An unexplained and persistent elevation of liver enzymes after the infection has cleared, however, may point to drug-induced liver injury. Good communication between the patient and healthcare provider is the best way to interpret these results accurately.

Comparison of Antibiotic Classes and Potential Blood Test Effects


Antibiotic Class	Examples	Potential Blood Test Effects
Cephalosporins	Cefalotin, cefazolin, ceftriaxone, cefoxitin	Increased creatinine (assay interference or kidney toxicity), false-positive Coombs test, elevated bilirubin, mild urine protein increase.
Penicillins	Amoxicillin, amoxicillin-clavulanate, piperacillin	Increased glucose (false positive with amoxicillin), elevated liver enzymes (amoxicillin-clavulanate), neutropenia, thrombocytopenia, hemolytic anemia (positive Coombs test).
Fluoroquinolones	Ciprofloxacin, levofloxacin, moxifloxacin	Altered glucose levels, elevated liver enzymes, false-positive urine drug screen for opiates.
Macrolides	Erythromycin, azithromycin	Elevated liver enzymes (AST, ALT), cholestatic hepatitis.
Sulfa Drugs	Trimethoprim-sulfamethoxazole	Increased creatinine, neutropenia, thrombocytopenia.
Glycopeptides	Vancomycin	Neutropenia (low WBC count).
Rifamycins	Rifampin	False-positive urine drug screen for opiates, liver dysfunction.

Navigating Bloodwork While on Antibiotics

To ensure the most accurate and reliable lab results, and to assist your doctor in proper interpretation, follow these key steps:

Inform your doctor: Always provide a complete and accurate list of all medications, including antibiotics and supplements, you are taking before any lab test.
Time tests strategically: For tests like blood cultures, it is crucial to draw blood before starting antibiotics, as the drugs can inhibit bacterial growth in the culture and cause a false negative. For other routine tests, delaying until after the course is finished and the drug has cleared your system can provide clearer results.
Wait after treatment: For non-critical blood tests, waiting at least 48 hours after the last dose of an antibiotic like amoxicillin is recommended. For tests assessing liver or kidney function after potential antibiotic-related injury, the wait time may be longer as recommended by your doctor.

Conclusion

Yes, antibiotics can and do interfere with bloodwork, either directly through interaction with lab assays or indirectly through the drug's effect on the body and the body's response to infection. This can result in elevated liver enzymes, altered kidney function indicators, and changes in blood cell counts. The key to accurate interpretation is open communication with your healthcare provider. By providing a full medication history and following guidance on the timing of tests, patients and doctors can work together to ensure that lab results provide a true picture of health and help avoid misdiagnosis.

For more information on drug-induced liver injury, the National Center for Biotechnology Information provides an extensive resource: Amoxicillin - LiverTox - NCBI Bookshelf.

Frequently Asked Questions

Yes, you can have some bloodwork done while on antibiotics, but it depends on the specific tests. For example, blood cultures for a bacterial infection should always be drawn before starting antibiotics. For routine tests, your doctor may recommend waiting until after your course of medication is finished to ensure the most accurate results.

For routine blood tests, it's generally advisable to wait at least 48 hours after your last antibiotic dose, as some antibiotics are out of the body within 24 hours. For complex tests or if you experienced side effects, your doctor may suggest a longer waiting period, such as two weeks, especially if you were unwell.

Common antibiotic classes known to affect lab tests include cephalosporins (can cause false positives), penicillins (associated with neutropenia and liver changes), and fluoroquinolones (can alter glucose levels and liver enzymes).

Yes, many antibiotics can cause elevated liver enzymes (AST, ALT) and bilirubin, which are measured in liver function tests. Amoxicillin-clavulanate is a particularly frequent cause of drug-induced liver injury.

While rare, some antibiotics can cause false-positive results on certain drug screens. For instance, rifampin, used to treat tuberculosis, has been known to trigger a false positive for opiates in some tests.

Some antibiotics, like penicillins and vancomycin, can temporarily cause a low white blood cell count (neutropenia) as a side effect. Alternatively, the low count may reflect the resolution of the infection, as the body's immune response subsides.

The most effective way is to maintain open communication with your doctor. Always disclose all medications you are taking, and if possible, schedule non-critical blood tests for a few days after finishing your course of antibiotics to allow your body to stabilize.