Answering Patients’ Most Common Questions About Lab Tests

Patients come into hospitals and clinics as people. People who do not know what the lab does, how tests are run, what the different tube colors mean, and what the results really mean. It is the job of laboratory professionals to do better explaining to them not only what they need to do, but also why. If the patients know they are part of the process, they are more willing to comply and make everyone's life easier.

Can I eat or drink before have blood drawn?

Fasting is typically required for tests that measure substances in your blood that are influenced by what you have eaten or drunk recently. Common tests that require fasting include:

1. Fasting Blood Glucose: Used to check blood sugar levels for diagnosing diabetes or monitoring glucose control. 4 hours should be long enough to reach baseline glucose levels.

2. Lipid Profile (Cholesterol and Triglycerides): Measures total cholesterol, LDL, HDL, and triglycerides, which can be affected by recent food intake. Different providers have varying standard for length of fasting, but traditionally the standard is 8-12 hours.

3. Basic Metabolic Panel (BMP): Includes measurements of glucose, calcium, and electrolytes, which can be influenced by food.

4. Iron Tests: Iron levels can vary depending on your last meal.

5. Liver Function Tests: While fasting isn’t always required, some doctors may recommend it for more accurate results.

Can I drink water before a lab draw?

Yes, and you should drink water. Dehydration can make it difficult for a phlebotomist to get blood because the veins won't be full and juicy. Dehydration can also affect some results artificially.

How long will results take?

The answer is “it depends”...

For routine blood tests, meaning CBC, BMP, CMP, Lipids, A1C and other common tests, the results are generally done in-house or in a close by facility where testing is done around the clock. If the testing is in-house, most routine testing should be done within 2 hours. This is also true with routine urine testing such as urinalysis or pregnancy tests.

For rarer, more specialized testing, the samples will need to be “sent out” to a reference laboratory. This adds time for transport. Send out testing can take a week or more, but as little as 24 hours if the reference lab is close by.

Any bacterial or fungal cultures that need to be worked up by the microbiology laboratory such as urine cultures or wound cultures, preliminary results are usually available after 24 hours and a final result may be ready by 48 hours. However, some cultures may be complicated and take greater than 48 hours, even up to a week.

Blood cultures, if negative, take 5 days to receive a final result. However, the blood cultures in automated systems can flag as positive at any time, often around 24 hours. If you do not have a positive result, it means the culture is still negative.

Mycobacterial testing, also known as acid-fast bacillus (AFB) or tuberculosis (TB) testing, often takes up to 6 weeks for a result. These organisms are very slow growing and take a very long time to ID and perform sensitivity testing for this reason.

Many kinds of testing such as STD testing will take up to a week, as they may be sent out or batched to be run on certain days of the week.

Biopsies will need to be taken to and reviewed by a pathologist. This can take anywhere from 24 hours to a week, depending on the complexity and the availability of the pathologist.

How Do I Know If My Results are Abnormal?

Lab results are “abnormal” if they are outside of the “reference range”. The reference range is established by running large numbers of "normal" patient samples.

Remember that the physician is not just looking at the results by themselves. Normal variation along a bell curve means that roughly 1 in 20 “normal” patients will still be outside of that range. There is variation in both patient results AND in testing. This is why clinical context matters and a test result does not exist in a vacuum.

Can I have them re-test if I don't trust the results?

Re-testing the same sample will give the same results. Bring up your concerns with the physician and if they agree, they can order a re-draw and re-test.

Keep in mind, lab personnel spend great effort to ensure accurate results. They test known samples daily to ensure that analyzers are on-target. Errors, if they occur, can be the result of improper collection, patient mix-ups, or random testing error.

Patients advocating on their own behalf is always a good idea. Any concerns should be answered.

How can I find better information about testing?

Anyone can call the laboratory and ask questions. If more technical information is needed, ask to speak with a tech (most people answering the phone are not technically trained).