How Medications Can Affect Laboratory Test Results

How Medications Can Affect Laboratory Test Results

The medications that a patient is taking can have a direct impact on some laboratory test results. So, care must be taken when interpreting results. It is always wise to record the medicines a patient is taking on the lab test request form. Also, need to check this whether they may influence results.

The results are as follows-

  • Anti-hypertensives which are being taken when detecting secondary causes of hormone replacement therapy
  • Or hypertension when asking for endocrine tests.

Medicines can also significantly impact the laboratory test process, or sample and cause inaccuracy in results. For example, a false negative of faecal antigen test may be seen in patients who are taking proton pump inhibitors or antibiotics when testing for H. pylori. It would reduce its gastric pressure.

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Fig: How Medications Can Affect Laboratory Test Results

Biological effect is another matter of concern when taking medications during lab tests. It causes an alternative test and affects the patient’s health badly.

Using PPIs or metformin for a long-term is a common explanation for the low level of vitamin B12. A lot of medicines affect the balance of potassium and sodium in our body.

When keeping in track of serum level or effect of medicine, it is important to decide the time of lab test according to drug’s metabolism. For example, the blood sample should be collected for lithium levels, at least 10 to 14 hours after the last dose.

A sample for digoxin testing must be collected 8 hours after taking the last dose of medicine. INR levels must be sampled every day in the morning and after taking a dose of warfarin in the evening, on a patient taking warfarin. It is important to count main changes in dosage.

Can I Take Medication before Blood Test?

The answer to this question depends on medication you take and the type of blood test you are going for. If you have any doubt regarding blood test or you are not sure about it, ask your healthcare provider for the proper advise. If you are taking drugs, never stop taking them unless your healthcare providers advise you to do so.

Laboratory Test Results and Medication

Some drugs can have a surprising effect on the laboratory test results. But it doesn’t actually mean you should stop taking medications. For instance, oral corticosteroids can raise the levels of your cholesterol in the test. However, your doctor can consider the same during analysis of laboratory test results. So, there is no need to stop taking medicine.

If you don’t know, keep taking medicine as prescribed by the doctor and ask your doctor. You may also have to take medicine to the professional laboratory tests. Vitamins, herbal remedies or supplements can also affect laboratory test results. If you take any medication, tell your doctor.

Should you stop taking medicine?

If you are taking prescribed medicine, never stop taking it unless your doctor advises you to do this. You may need to stop taking medicine in some cases before blood work.

Testing Medications

If you are going through a blood test to find out whether your medication is doing well, you must keep taking medicines in most cases. For example, if you take medicine to control cholesterol, keep taking it before going through blood work. It will show whether your medicine is doing well.

Are You Taking Thyroid Medications? How to Get Accurate Tests?

There are many people facing the problem after reducing thyroid medications due to faulty lab test analysis. We can hear a lot of complaints regarding that.

Example- On a routine of T3 and T4 drugs, all of these people were feeling good. Then they found low TSH levels on a lab test, which required their doctor to lower the drugs ratio. Doctors worry when they see even a slight rise of hyperthyroidism because of its effects on bones and heart. Severe problems can be very dangerous to a patient.

Dose needs to be just right to feel optimal because thyroid hormones are really ‘Goldilocks’. Doses which are too low or too high can cause symptoms. Whenever low TSH count is found in someone taking thyroid medicines, the doctor may ask “Did you take your drugs before drawing sample?” According to the timing of taking medicines and its type, the TSH level of a person may not be accurate.

Lab Tests Recommended in Thyroid

When starting treatment initially with thyroid medications to control hypothyroidism levels, it is recommended to test thyroid function. Also, the duration should be around 4 to 6 weeks after starting treatment to ensure the right dose of medication. It is important to run such tests more often and sooner if the person was facing symptoms of hypothyroidism severely.

There are three main tests to detect response to medications –

  • Free T3/Free T4 – In these tests, levels of an active hormone in the body are measured. When TSH tests are in normal range but there are low levels of T3 or T4, your physician may suspect central hypothyroidism, which is a rare type of hypothyroidism.
  • TSH – This pituitary hormone responds to high/low amounts of circulating hormone of the thyroid. This lab test is elevated in advanced cases of primary hypothyroidism. TSH will be low in Graves’ disease.

If these tests are not ordered by the doctor, make sure to order them yourself, even if you need to pay extra. On the timing of the sample draw, the rules are slightly different, according to the type of thyroid drug you take.

T4 Medications

T4 is having a half-life of up to 5 to 9 days. It means it takes up to 9 days for 50% dose to clean up the body once you get stable with it. T4 gets released slowly and steadily and it won’t produce much valleys and peaks which may feel like you are on thyroid low or high.

Whether T4 taken just before lab tests or you haven’t taken the same for around two days, the TSH value must be similar. So, you will get accurate TSH value whether or not taking T4 medicine before the test.

The same thing stands for free T3 levels when taking T4 medications. The free level of T3 will also remain constant. However, free T4 levels will show up high two hours after taking T4 medication.

Suppose you take T4 drugs at 8 am and undergo blood test after 2 hours, your T4 levels may elevate falsely in your thyroid lab tests. This way, your doctor may suspect high levels and lower your medication. Actually, your T4 may be in range for the day, with exception of 2 hours, after dose peak.

Taking T4 drugs in the morning before lab test will never be a problem in most cases. A lot of doctors change dosages as per the TSH. Hence it may remain stable after taking the T4 dose. However, to get reading of T4 levels which can reflect most days, you may need to postpone your drug until after the test.

Combination Medication

If you are taking T3-based combination drug, the timing of test matters and may make a significant change in getting the right dose of medication. It is because T3 has a half-life of just 18 hours to around 3 days, vary by person to person. It’s less than a day to start declining in the body rapidly in some people.

If you are taking a T3 dose recently, it may also affect your free T3 levels. After getting the dose, the T3 levels may raise and remain at peak for 4 hours after taking the dose.

It means if you had to take thyroid medication just before getting a thyroid test done, you might be seen overdosed in your lab tests. When you are under-dosed, it may show that your dose is in the normal range.

Hence, it is usually a good suggestion to delay thyroid medication until you get lab tests done properly. It is suggested to undergo thyroid test first in the morning, bring your drugs, and take them just after getting your thyroid test to get the right laboratory test results.

Keep in mind that half-life of drugs may vary by person to person. Some cases may have higher TSH levels, even in the state of euthyroid, when taking T3 medications. This is when you need to undergo T3 and T4 testing, and also watch out for symptoms.

Other Factors affecting Laboratory Test Results

Medications are not the only culprits behind false laboratory test results. So, let’s discuss some of the common factors which may affect your laboratory test results –

Diet and Nutrition

Calorie control, fasting, malnutrition, poor diet, and dehydration can affect ranges in lab tests. In some lab tests, the signs depend on some dietary factors.

Example– Ensuring proper gluten in your diet for up to several weeks before diagnosing celiac disease or fasting before determining the effect of the intervention in the patient with high levels of triglyceride.

In other cases, determining diet factors can interpret lab results which were not expected.

For example– A vegan diet can cause low vitamin B12 levels, high protein diet can cause higher uric acid levels, and a low carbohydrate diet can raise ketone levels.

Fasting for up to 12 hours before lab testing may help or even important, according to the clinical situation to get the most accurate outcome of the following tests, which are affecting by taking some foods –

  • Glucose – HbA1c is now suggested for some patients as a test of the first choice for testing and diagnosis of type 2 diabetes and it doesn’t need fasting.
  • Uric acid – The suggestion of fasting is usually not to get the right results but effect of dietary change may help figure out unexpected outcomes.
  • Triglycerides – Fasting is not for most patients to conduct lipid testing. But it may help to monitor in high triglyceride levels.
  • Creatinine – If you are taking the mean high meat meal, it can have a huge impact on serum creatinine.

Starvation and constant low-calorie intake can cause several changes to lab number like thyroid function, glucose, liver function, electrolytes, lipids, and renal function. Uric acid levels may elevate due to ketonemia (which is causing low clearance).


It can be the cause of potassium and sodium imbalances and affect several indices, such as-

  • Urea and creatinine
  • Lipids
  • Albumin
  • Hematology indices.


It is having various impacts on lab results, according to the nature of the nutritional status of the patient. We can consider Malnutrition as a deficiency of energy and protein, with or without deficiencies of micronutrients. However, malnutrition may classify as over-nutrition, under-nutrition, or lower ferritin, vitamin B12 levels, and folate levels.


The consumption of caffeine causes the transient rise in levels of blood glucose and can affect glucose tolerance. It can also affect other investigations, including testing of metanephrines for the investigation of hypertension.


Exposure to nicotine and constant smoking can have both chronic and acute effects on lab tests. Within just one hour of smoking up to 5 cigarettes, serum/plasma concentrations of adrenaline, fatty acids, aldosterone, glycerol, and cortisol may raise. People who are heavy smokers may have significant rises in heavy metals, leukocytes, tumor markers, lipoproteins, and hematocrit (PCV), and lower activity of some enzymes.


On laboratory parameters, the effect of exercise relies on the health condition of a patient. Vigorous or extreme exercise in a person who is not familiar to this level of activity can make changes to some lab parameters.

For instance, exercise is the most common cause of the high levels of creatine kinase (CK). Heavy exercise can increase CK levels for around a week. CK levels almost remain beyond levels in athletes and well-muscled individuals.

Thyroid function can also change in people going through the intense exercise. For example, anaerobic exercise raises FT4 and TSH levels but reduces FT3 levels. Liver function (ALT or AST) tests can raise the levels post exercise.

APTT or activated partial thromboplastin time and fibrinogen may decrease. A lot of these effects may transitory (for example, persistent for few hours to days after exercise), but it relies on patient factors.

Alcohol consumption

On lab findings, the effect of alcohol intake depends on the extent and duration of use. Transient or acute effects of alcohol consist of reduced serum glucose within 2 to 4 hours and raised levels of plasma lactate with lower excretion of urinary uric acid.

Here are the chronic impacts of consuming alcohol on lab testing –

  • High triglyceride levels
  • The higher levels of mean cell volume (MCV) and glutamyl transferase (GGT) which are effect of high intake of alcohol.
  • The higher ratio of AST/ALT.
  • The level of CK because of the myopathy of alcohol.
  • High ferritin and uric acid levels because of alcoholic hepatitis and fatty liver
  • Other hematological abnormalities.

Alcohol intake is also the reason for deficiency of minerals and vitamins. It is because of replacement of food with alcohol or interference of absorption of minerals and vitamin.

Analytical Changes

Analytical changes take place because of old equipment and improper testing methods. Also, it may lead to slight changes in analyte values every time while the testing. With modern lab equipment and testing methods, analytical variation is usually less common in varying laboratory test results. Basically, the variation in testing must be less than half the biological variation of analyte (CVi).

Storage, Collection and transit of samples

During the collection of the sample at the lab, you need to know the medium of sample collection and type of container needed by the lab. It can affect the laboratory test results. For example, in case of PCR testing, a swab for pertussis must be moved in a tube with the universal medium of viral transport or a dry tube, but not in a tube that has charcoal transport medium.

For optimal laboratory test results, here are some of the common examples of transport or collection–

  • Urine samples for culture must be stored in the refrigerator before transit to lower multiplication rate of microorganisms.
  • For glucose analysis, samples must be separated just after collection. It should be done even with samples collected in oxalate or fluoride collection tubes, as lower glucose still can take place for up to 60 to 90 minutes.
  • Samples for phosphate should not leave all night. As results can be altered, for example, collection in the late evening with delayed sample transport to the lab.
  • Blood samples collected for studies, such as D-dimer, platelet count, APTT, fibrinogen, and prothrombin time, should be moved to the lab within four hours after collecting it.
  • Fecal samples for culture should transit preferably to the lab within four hours.
  • Samples for semen for fertility testing must be stored at body temperature by the patient, for example, by storing in the clothing pocket and transported to the lab within one hour.

Use Lab Me Analytics for Accurate Results

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