How do you read your blood test results?

What a reference range is

A reference range is the span of results expected in healthy people. MedlinePlus explains that ranges are based on test results from large groups of healthy people, and can differ between groups such as children and adults. For many tests, the range covers the middle 95% of that group (testing.com), leaving out the lowest 2.5% and highest 2.5% of values (Journal of the American Board of Family Medicine).

That method has a built-in consequence. By definition, about 5% of healthy people, roughly 1 in 20, will have a result outside the range on any one test. On a 20-test panel, 64.2% of healthy people would have at least one result flagged as abnormal. The reverse also holds: a result inside the range is not a guarantee of good health (MedlinePlus).

Each laboratory establishes or validates its own ranges, so judge a result against the range from the lab that ran it. Age and sex are the most common reasons ranges differ, and labs can set separate ranges for pregnancy or fasting samples (testing.com).

Some tests use decision limits rather than ranges: a fasting glucose of 126 mg/dL or above, on more than one occasion, indicates diabetes (testing.com). A doctor reads results like these against the limit, not against the spread of healthy people.

Flags and units

Lab reports draw attention to results outside the reference range. An H next to a result may mean it is higher than the range, an L may mean low, and WNL usually means within normal limits. A flag is a prompt to look closer, not a verdict. A critical value, one that may signal a life-threatening problem, is different: the lab must immediately alert whoever ordered the test (testing.com).

Units can vary from lab to lab, the way a weight can be written in pounds or kilograms (testing.com). A number only means something next to its unit and its range, so compare results from the same lab where you can.

Every marker on the Wolfburgh panel has its own guide at https://www.wolfburgh.com/biomarkers, covering what the marker is and what high and low results mean.

Why one result outside the range is not a diagnosis

A result outside the range may or may not be a sign of a health problem, and healthy people sometimes have results outside it (MedlinePlus). Some out-of-range results resolve on their own, especially near the border of the range, and your provider will most likely want to rerun the test (testing.com).

A doctor reads each result in context: your history, symptoms, medicines and other results. Tests can also be wrong in both directions. A false positive shows a condition you do not have, and a false negative misses one you do (MedlinePlus). When a result may be inaccurate, the usual answer is another test.

Values the lab calculates

Not every number on a report is measured directly. Many labs calculate LDL cholesterol with the Friedewald equation: total cholesterol minus HDL, minus triglycerides divided by 5, all in mg/dL. The triglycerides divided by 5 term is itself an estimate of VLDL cholesterol. The equation is not valid when triglycerides are above 400 mg/dL, and the 2018 US cholesterol guideline recommended the newer Martin-Hopkins equation, so methods vary by lab (CDC).

Ratios, such as total cholesterol to HDL, divide one measured result by another. Calculated values inherit the behaviour of their inputs: if triglycerides swing, a calculated LDL and VLDL swing with them. The Wolfburgh panel has 17 markers: 13 measured biomarkers plus 4 values the lab calculates from them.

Why trends matter more than one result

One result is a snapshot. A rise or drop in a marker can matter even when every value stays within the range, and providers may order follow-up tests some months later to check you are on track (testing.com).

Trends are only fair when tests are comparable. Results vary between laboratories even with the same methods and instruments, while repeat testing within one lab varies much less, which argues for using the same lab for serial monitoring (Journal of the American Board of Family Medicine). Keep the same lab and fasting state where you can, so a change reflects you rather than the conditions.

What can skew a result

Several everyday factors move results without any change in your health. Tell whoever orders the test about any of these.

• Food and fasting: what you eat and drink can affect results (MedlinePlus). Follow the preparation instructions you were given. Going without food for 24 to 48 hours can lower albumin.
• Hard exercise: exercising hard before a test can change results (MedlinePlus).
• Illness: even a mild cold, infection or injury can spike hs-CRP, so a high result is confirmed with a repeat test, ideally two weeks later, when you are well (testing.com).
• Biotin: the FDA warns that biotin can significantly interfere with certain lab tests and cause falsely high or low results, with hormone tests such as TSH among those studied. Beauty-focused supplements can contain 5,000 to 10,000 mcg, against a recommended daily intake of 30 mcg for adults (testing.com).
• Hydration: a higher than normal albumin may be a sign of dehydration (MedlinePlus).
• Medicines and cycles: some medicines, and having a menstrual period at the time of the test, can affect results. Never stop a medicine or supplement to change a result; tell your doctor what you take.

When to speak to a doctor

Talk to your doctor about what your results mean for you, even when they are normal (MedlinePlus). Speak to them promptly if:

• you are told a result is a critical value
• a result is outside the range on more than one test, or keeps moving in one direction
• a result is well outside the range rather than just over the border
• you have symptoms, whatever the numbers say