Which blood tests are worth knowing

A complete blood count (CBC) measures red cells, white cells and platelets. It can point to anemia, infection or problems with the bone marrow.

A comprehensive metabolic panel (CMP) covers kidney and liver function, electrolytes such as sodium and potassium, blood sugar, calcium and protein levels.

A standard lipid panel reports total cholesterol, LDL, HDL and triglycerides. It is the usual starting point for estimating heart disease risk.

ApoB counts the number of particles that carry cholesterol into artery walls. Because each harmful particle carries one ApoB, many clinicians use it as a more direct measure of risk than LDL alone.

Lp(a), or lipoprotein(a), is a type of cholesterol particle whose level is set mostly by your genes. It changes little over a lifetime, so for most people one test is enough. A high level raises heart disease risk even when other numbers look fine.

HbA1c reflects your average blood sugar over the past two to three months. It is used to screen for prediabetes and diabetes.

Fasting glucose is a single reading of blood sugar after not eating overnight. Fasting insulin, read alongside glucose, can show insulin resistance before blood sugar starts to rise.

High-sensitivity CRP (hs-CRP) picks up low levels of inflammation, which is linked to heart disease risk. It also rises with a cold, an injury or a hard workout, so one high result should be repeated when you are well.

TSH is the pituitary's signal to the thyroid. A high TSH usually means the thyroid is underactive, and a low TSH can mean it is overactive. Free T4 measures the main thyroid hormone itself and helps make sense of an abnormal TSH.

Vitamin D is measured as 25-hydroxyvitamin D. Low levels are common in people who spend little time outdoors or live far from the equator.

Ferritin reflects your iron stores, and an iron panel shows how much iron is moving in your blood. Low ferritin is a common cause of fatigue, especially in people who menstruate. Ferritin also rises with inflammation, so a normal number does not always rule out low iron.

B12 is needed for nerves and red blood cells. Levels can run low with a vegan diet, with age, and with some medications such as metformin and acid reducers.

Testosterone, estradiol and related hormones are worth checking when there are symptoms or a specific question, such as low energy, low libido or irregular cycles. Testosterone is usually drawn in the morning, and women's hormone levels change across the menstrual cycle, so the day of the draw affects how the result reads.

Many markers move with sleep, recent exercise, illness, alcohol and what you ate the day before. A baseline taken when you are well gives you something to compare against.

When you retest after a change, keep conditions the same: same lab if you can, same time of day, and fasting if the first test was fasting.

Hundred offers lab testing at hundred.com/thesystems. Hundred is a separate company from The Systems, with its own process, pricing, privacy policy and terms. Check those on its website before you order. Following a link from this site does not order a test or arrange a review of your results.

Reference ranges come from population averages and do not account for your age, sex, symptoms, medications or family history, which all change what a number means.

Go over your results with a clinician who knows your history before you change medications, supplements or your diet because of them.