How to read your blood test results

Question 1

How do I read my blood test results?

Accepted Answer

The reference range listed on your own lab report is the one to read a result against, and its units matter too. A reference range is a set of numbers marking the high and low ends of the results considered normal for that test. Results come in different forms, MedlinePlus says: some are a number, such as 125 or 80, and others say whether something was found.

Your provider reads results together with your physical exam, health history, family health history and any imaging, and in most cases that combination leads to a more accurate diagnosis than any one lab test, MedlinePlus says. Seeing a result in your electronic health record doesn't change that: it's still important to talk with your provider to find out what it means for you. The FDA puts it this way: results must be interpreted within the context of your overall health and should be used along with other exams or tests.

Question 2

What does it mean when a result is outside the reference range?

Accepted Answer

Outside the range isn't the same as having a problem. A result higher or lower than the range that applies to you isn't always a sign of a problem, MedlinePlus says, and healthy people sometimes have results outside it. When a result is outside the range, your provider will look at other information about your health to understand what may have affected it.

The reverse holds too. A result inside the range isn't always a guarantee of good health, and some people with health problems have results that count as normal. Additional testing can still be needed, especially with symptoms.

Results are reported in different ways, MedlinePlus says. The FDA describes some tests as showing whether your results fall within normal ranges. Neither of those pages says what a particular mark, such as a letter or an arrow, stands for, so a fair question is whether your report has a key for any mark beside a result, and what it says.

Question 3

Why do reference ranges differ from lab to lab?

Accepted Answer

Labs often use different testing methods, so they often use different reference ranges, and the range that counts is the one printed on your own report. MedlinePlus says results can't be set against ranges found elsewhere, which is why its own test pages give no normal range for most tests.

Ranges also differ by group. A test may have separate ranges for groups such as children and adults. Normal values are usually given as a range because they vary from person to person, and the FDA lists sex, age, medical history, general health and variation from one laboratory to another among the factors that can affect results.

Question 4

Why do units matter when I read a result?

Accepted Answer

Tests are measured in different units, so a number only means something in the units it was reported in. MedlinePlus compares this to measuring recipe ingredients in teaspoons, cups or ounces, and its examples include micrograms per deciliter (mcg/dL), micromoles per liter and picograms per milliliter (pg/mL).

When you hold two reports side by side, a fair question for the lab or your provider is whether both used the same units.

Question 5

What can change a result before it reaches the lab?

Accepted Answer

The accuracy of certain test results can be affected by preparation and the day of the test. Factors can include certain foods and drinks, certain medicines or supplements, hard exercise before the test, and whether preparation instructions, including fasting, were followed, according to MedlinePlus. A small departure from the instructions can still matter a great deal for the result.

Most blood tests need no special preparation. When a fast is asked for, the reason is that nutrients from food and drink get into your bloodstream and can change the levels of various substances in your blood. The fasting page at MedlinePlus gives 8 to 12 hours as the usual length and says your provider will tell you exactly how long for your test.

A fair question for your team is whether anything about how you prepared, such as a fast, a hard workout or a medicine, could have mattered for a particular result.

Question 6

How do I follow the same test over time?

Accepted Answer

MedlinePlus advises trying to stay with the same lab when you're following results over time, and says you can't compare test results from different labs. With results from more than one lab, a fair question is whether each number is being read against its own range and units.

Labs sometimes compare your results with earlier tests to see if there has been a change in your condition, the FDA says.

Question 7

What do the words negative, positive and inconclusive mean?

Accepted Answer

Some results are words instead of numbers. "Negative" means the test did not turn up what it looked for, and "positive" means it did. If a result is inconclusive, meaning it wasn't clearly positive or negative, you will probably be tested again, MedlinePlus says.

Test results are usually accurate, but no test is perfect, and false results don't happen often. They are more likely with certain types of tests or if testing wasn't done right.

Question 8

What could I ask about a result?

Accepted Answer

You could ask which range your lab used and whether the units are the ones you expect, whether anything about the day of the test could have affected the result, whether it is worth repeating or needs other tests, and how it compares with earlier results from the same lab.

This page can't say what a particular number means for a particular person.