Start with the range printed beside each result. It's built from the results of a large group of healthy people, so it shows what's common, not what's right for you (MedlinePlus). Ranges usually cover the middle 95% of healthy people, which means 5% of healthy results get flagged (Ozarda, 2016).
Key takeaways
A reference range usually covers the middle 95% of results from healthy people, so 5% of healthy results fall outside it (Ozarda, 2016).
Labs often use different methods and different ranges. Read your result against the range on your own report (MedlinePlus).
A result outside the range may or may not be a sign of a problem. A result inside it isn't a guarantee of good health (MedlinePlus).
Your own earlier results can show a change that a population range misses (Badrick, 2021).
How do I read my lab results?
Read each result against the range on the same report. That range comes from testing healthy people, and it usually covers the middle 95% of their results (Ozarda, 2016). Then look at the units, any flag, and how the number compares with your earlier results. Your doctor reads it with your exam and history (MedlinePlus).
A typical line on a report has four parts:
Part | What it tells you |
|---|---|
Test name | What was measured, often shortened, like LDL for a type of cholesterol |
Result | Your number |
Units | How it was measured, like micrograms per deciliter |
Reference range | The low and high ends of results considered normal at that lab |
Each of those parts is described in the MedlinePlus guide to lab results (MedlinePlus).
What's a reference range?
A reference range is the span of results a lab considers normal. The lab tests a group of healthy people and lines up their results. The low end is usually the 2.5th percentile and the high end the 97.5th (Ozarda, 2016). So the range covers the middle 95% of healthy people.
A guideline recommends at least 120 people when setting a range, so the lab can gauge how precise its limits are. Ranges should also be split by age and sex at the very least (Ozarda, 2016).
A test can have separate ranges for different groups, like children and adults (MedlinePlus). That's why two people can get the same number with different flags.
Why do ranges differ between labs?
Labs often use different testing methods, and different methods give different ranges (MedlinePlus). Each range usually covers the middle 95% of one group of healthy people (Ozarda, 2016). So compare your result only with the range on your own report.
You can't compare your result with a range you find online, or compare results from different labs directly. If you're following a trend, try to use the same lab each time (MedlinePlus).
Standards also say each lab should check its own ranges from time to time (Ozarda, 2016). A lab that changes its method may change its range too.
What does a high or low flag mean?
A flag means your result fell outside that lab's range, where about 5% of healthy results land (Ozarda, 2016). It may or may not be a sign of a problem. It's common for healthy people to sometimes have results outside the range. When that happens, your provider looks at other information to understand what may have affected it (MedlinePlus).
The opposite is also true. A result inside the range isn't always a guarantee of good health. Some people with health problems have results in the normal range (MedlinePlus).
How often do healthy people get a flag?
On any one test, about 1 in 20 healthy people get a flag, because ranges leave out 5% of healthy results (Ozarda, 2016). Panels run many tests at once, so the chance of at least one flag rises fast. The table below shows the simple math, treating each test as unrelated to the others.
Tests on your panel | Chance a healthy person gets at least one flag |
|---|---|
1 | 5% |
5 | about 23% |
10 | about 40% |
20 | about 64% |
The math is 1 minus 0.95 to the power of the number of tests. Real tests are often linked, so this is a rough guide, not a measured rate. The point stands: one flag on a long panel is common.
Is every result read against a range?
No. Some results are read against a decision limit instead. That's a cutoff set from clinical studies, marking the point where a doctor takes a specific step (Ozarda, 2016). A1c is one example: 6.5% or above is the level used to diagnose diabetes (NIDDK). There's more in what your A1c number means.
Other results aren't numbers at all. A negative result means the test didn't find what it looked for. A positive one means it did. An inconclusive result means it wasn't clear, and you'll probably be tested again (MedlinePlus).
What can change a result?
Food and drink, some medicines or supplements, hard exercise, a menstrual period and skipped fasting can all change a result (MedlinePlus). That's why some tests come with prep instructions. Before a fasting glucose test, you fast for at least 8 hours (NIDDK). Tell your provider about every medicine, vitamin and supplement you take (MedlinePlus). Here are the common ones:
What you ate or drank before the test
Medicines or supplements you take
Exercising hard before the test
A menstrual period at the time of the test
Whether you followed the prep instructions, including fasting
Your body also changes. Blood sugar moves up and down when you eat or exercise, and sickness and stress can affect it. For blood sugar tests, even the same sample tested again in the same lab can give a slightly different result (NIDDK).
That's one reason a single result tells you less than a few. For blood pressure readings, see the white coat effect.
Why compare your results with your own past results?
Because a range covers the middle 95% of many healthy people, and people differ (Ozarda, 2016). You can sit inside it and still have had a real change. A review that uses kidney function as its example says comparing with your previous results may be needed. The ideal, it says, is a range built for one person (Badrick, 2021).
That's the same reason we read your other numbers against what's typical for you. Our methodology explains how, and why we compare you to you covers the research.
When should I talk to my doctor?
Talk to your doctor about any result that worries you, flagged or not. About 5% of healthy results get flagged (Ozarda, 2016). Some people with health problems get normal results (MedlinePlus).
Lab tests don't give a full picture alone. Your provider reads them with your physical exam, your health history and any family health history you know (MedlinePlus). If your provider thinks a result may be wrong, you may need another test (MedlinePlus).
Questions that help:
Is this just past the edge of the range, or far outside it?
Could anything before the test have moved it?
Should we repeat it, and when?
How does it compare with my earlier results?
Some medicines can affect your result, so tell your provider about everything you take. Don't stop any medicine unless your provider tells you to (MedlinePlus).
Frequently asked questions
Why does my result say "normal" when I feel unwell? A normal result isn't a guarantee of good health, and you may need more tests if you have symptoms (MedlinePlus).
Can I use ranges I find online? No. Use the range on your own report, because labs often use different methods and ranges (MedlinePlus).
How should I prepare for a blood test? Follow your provider's instructions, including any fasting. Tell them about every medicine, vitamin and supplement you take (MedlinePlus).
Does a flag mean I'm sick? Not always. A result outside the range may or may not be a sign of a problem (MedlinePlus).
When your lab results reach Knit, they're in the Record tab, under Lab results, beside the rest of your health record. These articles share patterns, not diagnoses, so talk to your doctor about anything that worries you.
