How to read lab results without panicking

Almost every lab report has the same columns:

According to MedlinePlus, a reference range is worked out by testing a large group of healthy people, and it can differ between labs and by age or sex. So compare your result with the range printed on your own report, not with a table from the internet.

It is common for a healthy person to have a result slightly outside the range, and a person with a condition can have "normal" results. Many things shift a result: what you ate or drank, medicines and supplements, hard exercise before the test, your menstrual cycle, or not fasting when you were supposed to.

Tests that look for something specific may say negative (not found), positive (found) or inconclusive (needs repeating). No test is perfect: false positives and false negatives happen.

Complete blood count (CBC). Measures red blood cells, which carry oxygen; white blood cells, which fight infection; platelets, which help blood clot; hemoglobin, hematocrit and red cell size (MCV). Low hemoglobin can point to anemia; a high white count sometimes points to an infection. It usually needs no fasting.

Comprehensive metabolic panel (CMP). Measures 14 things: glucose, calcium, electrolytes (sodium, potassium, bicarbonate and chloride), proteins (albumin and total protein), liver enzymes (ALP, ALT and AST), bilirubin, and BUN and creatinine, which show how well your kidneys filter. High liver enzymes are a common reason to talk about fatty liver.

Lipid panel (cholesterol). Measures total cholesterol, LDL ("bad"), HDL ("good") and triglycerides. It may need a 9- to 12-hour fast. For adults, MedlinePlus gives healthy levels as total cholesterol under 200 mg/dL, LDL under 100 mg/dL and HDL of 60 mg/dL or more; your own target depends on your risk. See high cholesterol.

Hemoglobin A1C. Reflects your average blood sugar over the past 3 months. According to the CDC, below 5.7% is normal, 5.7% to 6.4% is prediabetes and 6.5% or above is diabetes. For people with diabetes the usual goal is 7% or less, but your doctor sets it. See also high blood sugar.

TSH (thyroid). A high TSH usually suggests an underactive thyroid (hypothyroidism), and a low one an overactive thyroid. If it is out of range, doctors usually order more thyroid tests.

Vitamin D (25-hydroxy vitamin D). According to MedlinePlus, this is the most accurate way to measure how much vitamin D you have. It is reported in ng/mL, and many experts recommend 30 to 80 ng/mL. It can be low from too little sun, too little vitamin D in your diet, trouble absorbing it, or some medicines. See low vitamin D.

Vitamin B12. Measures B12 in your blood. According to MedlinePlus, a low level can cause numbness or tingling in the arms and legs, weakness and loss of balance, and the test is used when a certain type of anemia is found. See vitamin B12 deficiency.

Hepatitis panel. A series of blood tests, for antibodies and antigens, that looks for a current or past infection with hepatitis A, B or C. A positive antibody does not always mean an active infection: the doctor explains what it means for you. See hepatitis testing.

Follow the instructions on your order and from the lab. Some tests, such as a lipid panel or a fasting glucose, may require several hours without food; others, such as a CBC, usually don’t. Tell your doctor everything you take, including supplements, because some change results. Don’t stop a medicine on your own for a test.

This page can’t tell you what your numbers mean: the doctor does that with your history, symptoms and medicines.

• If Dr. Diana Nieves Castro ordered your tests at an earlier visit, she reviews them with you in a US$49 follow-up visit. It is for existing patients only.
• If another doctor ordered the tests, or they come with a new problem, choose a US$79 doctor visit so the doctor can evaluate you fully.
• If you still need the tests, we give you a lab order for a lab near you.

We send you a secure video link; it works on your phone, tablet or computer. Have your report ready. If appropriate, the doctor prescribes and we send your prescription to your pharmacy.

If the lab or a doctor calls to tell you a result is urgent, follow their instructions. And if you have severe symptoms, such as chest pain, shortness of breath, confusion, fainting or bleeding, don’t wait for an appointment: call 911 or go to the ER.