How do I read a blood test report?

When you get your test report, your eye goes first to the numbers marked with a sign or a colour. Most people start to worry right there. This article explains how to look at the sheet without fear and without diagnosing yourself.

What does “reference range” mean?

Next to each number a “reference range” or “normal range” is written. According to MedlinePlus, this range comes from the test results of a large group of healthy people. Two points:

1. Each laboratory may have its own range. Compare the number with the range printed on that same sheet, not with the sheet of another laboratory. To see a trend, it is best to have tests done at the same laboratory.
2. Look at the units. For example, mg/dL is different from mmol/L, and comparing them is meaningless.

A number outside the range is not always an illness

MedlinePlus says healthy people too sometimes have a result outside the range. Things that change a result include: food and drink, medicines and supplements, heavy exercise before the test, the time of the menstrual cycle, and not following the preparation instructions such as fasting. On the other hand, a “normal” result does not always mean perfect health, and sometimes the assessment needs to continue because of symptoms.

That is why the doctor reads the number together with your symptoms, examination and history, not on its own.

A few common tests

Blood sugar and HbA1c. The CDC cut-offs: HbA1c below 5.7% is normal, 5.7 to 6.4% is prediabetes, and 6.5% or higher is in the diabetes range. Fasting blood sugar of 99 mg/dL or lower is normal, 100 to 125 is prediabetes, and 126 or higher is in the diabetes range. HbA1c is the average blood sugar over the past two to three months. A single high fasting blood sugar is not a diagnosis on its own; according to the CDC, the doctor may repeat the test to be sure.

Blood fats. For adults, MedlinePlus counts total cholesterol below 200 and LDL (“bad” fat) below 100 mg/dL as healthy, and HDL (“good” fat) of 60 or more as best, with below 40 in men and below 50 in women counted as low. The same source says the right level depends on age, family history, lifestyle and other risk factors. Ask the doctor for your own target. A fat test may need 9 to 12 hours of fasting (MedlinePlus).

Liver enzymes. ALT, AST and ALP are enzymes made mostly in the liver. MedlinePlus says abnormal results do not always mean a liver problem; medicines, inflammation, a bile duct problem or other causes can also be responsible, and the doctor looks for the cause using symptoms and history.

Blood cell count (CBC). Red cells and haemoglobin (which carry oxygen), white cells (defence) and platelets (clotting). MedlinePlus says changes in red cells can relate to dehydration, anaemia or iron deficiency, and changes in white cells to infection or other causes; a high or low number is not necessarily a disease.

TSH. It shows how strongly the gland in the brain is stimulating the thyroid. A high value usually goes with an underactive thyroid and a low value with an overactive one. Serious illness, age over 80 and the first three months of pregnancy may change the result (MedlinePlus). So comparing your number with other people’s or with a general figure found online does not help.

What should you do with the report?

• Take the report to the doctor. MedlinePlus stresses that you should talk to the doctor to learn what the result means for you, even if it is normal.
• Do not change a medicine because of your result. If your blood sugar, blood pressure or thyroid has become “normal”, it does not mean you should stop the medicine. If a result is high, do not add a medicine yourself either.
• Tell the doctor about all medicines and supplements. They can change the result.
• If you have serious symptoms, do not wait for the result or an appointment; see the guide “When should I call urgently?”.

Questions you can ask

• Which number matters most for me, and why?
• How is this number different from my earlier test?
• Does it need to be repeated? When?
• Could anything in my food, medicines or supplements have affected this number?
• What is my target for this test?

Every patient’s treatment plan is different; ask the doctor about your own situation.