Lab test glossary
Understand your lab results.
Plain-English explanations of common blood tests — what each one measures, what can push it up or down, and when it’s worth raising with your doctor. General education, not medical advice: your own results are for you and your clinician to interpret together.
Blood count
Hemoglobin
Hemoglobin is the oxygen-carrying protein in red blood cells; low levels define anemia, and high levels can point to dehydration or overproduction of red cells.
Hematocrit
Hematocrit is the percentage of your blood volume made up of red blood cells; it tracks closely with hemoglobin and shifts with hydration.
White Blood Cell Count
The white blood cell count reflects your immune cells; it rises with infection or inflammation and can fall with some conditions or medications.
Platelet Count
Platelets help your blood clot; very low or very high counts can affect bleeding or clotting risk.
Red Blood Cell Count
The red blood cell count measures the cells that carry oxygen; it's interpreted with hemoglobin, hematocrit, and red-cell indices.
MCV
MCV is the average size of your red blood cells — one of the most useful numbers for figuring out what's behind an anemia.
RDW
RDW measures how much your red blood cells vary in size; a high RDW can be an early clue to a developing or mixed anemia.
Neutrophils
Neutrophils are your most common white blood cell and first responders to bacterial infection; the count rises with infection and can fall from some medications or conditions.
Lymphocytes
Lymphocytes are the white blood cells central to fighting viruses and building immune memory; they often rise with viral infections.
Iron studies
Ferritin
Ferritin reflects your body's stored iron — low ferritin is one of the earliest signs of iron deficiency, and high ferritin can signal inflammation or iron overload.
Iron & Iron Saturation
Serum iron and transferrin saturation show how much iron is circulating and how full your iron-transport system is — read alongside ferritin.
Cholesterol & lipids
LDL Cholesterol
LDL is the cholesterol fraction most associated with plaque buildup in arteries; lower is generally considered better for cardiovascular risk.
HDL Cholesterol
HDL helps clear cholesterol from the blood; higher levels are generally considered protective against heart disease.
Triglycerides
Triglycerides are a type of fat in the blood; high levels often relate to diet, alcohol, and metabolic health, and very high levels carry their own risks.
Total Cholesterol
Total cholesterol combines LDL, HDL, and other fractions into one number — useful as a screen but less meaningful than its parts.
ApoB
ApoB counts the cholesterol particles that can lodge in artery walls — often a sharper measure of heart risk than LDL cholesterol alone.
Lipoprotein(a)
Lipoprotein(a) is a largely inherited cholesterol particle that independently raises heart risk — usually worth checking once in a lifetime.
Blood sugar
Hemoglobin A1c
A1c estimates your average blood sugar over the past ~3 months and is the standard test for screening and monitoring diabetes.
Blood Glucose
Blood glucose is the amount of sugar in your blood at the moment of the draw; whether you were fasting changes how it's read.
Fasting Insulin
Fasting insulin shows how hard your body is working to keep blood sugar normal — it can reveal insulin resistance years before glucose starts to rise.
Liver
ALT
ALT is a liver enzyme; elevated levels are one of the most common signs of liver inflammation or injury.
AST
AST is an enzyme found in the liver and muscle; raised levels can reflect liver or muscle injury, and it's interpreted alongside ALT.
Alkaline Phosphatase
Alkaline phosphatase is an enzyme from the liver, bile ducts, and bone; the pattern of change helps point to which of those is involved.
Bilirubin
Bilirubin is a breakdown product of red blood cells processed by the liver; high levels can cause the yellowing of jaundice.
Albumin
Albumin is the main protein made by your liver; a low level is a broad signal that can point to the liver, kidneys, nutrition, or inflammation.
Total Protein
Total protein sums albumin and the antibody-rich globulins; the balance between them helps explain an abnormal result.
GGT
GGT is a liver enzyme sensitive to bile-duct problems and alcohol; it's often used to clarify what's driving a high alkaline phosphatase.
Kidney
Creatinine
Creatinine is a muscle waste product cleared by the kidneys; it's a core measure of kidney function and feeds the eGFR calculation.
eGFR
eGFR estimates how well your kidneys filter waste, calculated from creatinine; lower values indicate reduced kidney function.
Blood Urea Nitrogen
BUN reflects kidney function and hydration; it can rise with dehydration or reduced kidney filtering and is read alongside creatinine.
Thyroid
TSH
TSH is the pituitary's signal to the thyroid and the primary screen for an under- or over-active thyroid — read counterintuitively, since high TSH usually means low thyroid activity.
Free T4
Free T4 is the active, unbound thyroid hormone available to your tissues, read alongside TSH to assess thyroid function.
Thyroid Antibodies (TPO)
TPO antibodies are immune proteins aimed at the thyroid; a positive result points to autoimmune thyroid disease such as Hashimoto's.
Free T3
Free T3 is the most active thyroid hormone; it's sometimes added to TSH and free T4 to complete the thyroid picture.
Hormones
Testosterone
Testosterone is the primary male sex hormone (also important in women); low levels can contribute to fatigue, low libido, and changes in mood and body composition.
Cortisol
Cortisol is your body's main stress hormone, and it follows a strong daily rhythm — which is why the time of the draw is everything.
Estradiol
Estradiol is the main form of estrogen, central to the menstrual cycle, menopause, and hormone therapy — read together with sex, cycle timing, and life stage.
Inflammation
C-Reactive Protein
CRP is a marker of inflammation that rises with infection, injury, or chronic inflammatory conditions — nonspecific but sensitive.
ESR (Sed Rate)
ESR is an older, slower-moving marker of inflammation, often read alongside CRP.
ANA
ANA is a screening test for autoimmune conditions; a low-positive result is common and often means nothing on its own.
Vitamins
Vitamin D
Vitamin D supports bone health and immune function; low levels are common, especially with limited sun exposure.
Vitamin B12
Vitamin B12 is needed for nerve function and red-blood-cell production; deficiency can cause anemia and neurological symptoms.
Folate
Folate works with vitamin B12 to make red blood cells; a deficiency causes a large-cell anemia and matters especially before and during pregnancy.
Other
Potassium
Potassium is an electrolyte vital for nerve and heart function; both high and low levels can affect heart rhythm.
Sodium
Sodium is an electrolyte that regulates fluid balance; abnormal levels usually reflect hydration or kidney/hormone issues.
Calcium
Blood calcium supports bones, nerves, and muscles and is tightly controlled by hormones and vitamin D.
PSA
PSA is a protein made by the prostate, used in prostate-cancer screening and monitoring — best interpreted as a trend and with shared decision-making.
Magnesium
Magnesium is an electrolyte your muscles, nerves, and heart depend on; low levels are common and can cause cramps and affect heart rhythm.
Bicarbonate (CO₂)
Bicarbonate reflects your body's acid-base balance; abnormal levels point toward breathing, kidney, or metabolic causes.
Chloride
Chloride is an electrolyte that tracks with sodium and helps maintain fluid and acid-base balance.
Uric Acid
Uric acid is a waste product that, at high levels, can crystallize in joints and cause gout — and it ties into kidney and metabolic health.
Homocysteine
Homocysteine is an amino acid tied to B-vitamin status; high levels are associated with cardiovascular risk and often reflect low B12, folate, or B6.