What Do Doctors Want You to Understand?

Blood work is part of many checkups. Our guide to what to expect at your annual physical covers the visit as a whole. This guide covers the blood tests themselves: what each one measures, and what a result outside the range really means.

Start with how "normal" is defined. Labs base each normal range, also called a reference range, on results from large groups of healthy people.² The range covers the middle 95% of those results.¹ That means about 5% of healthy people fall outside it, with nothing wrong.¹

The more tests you have, the more likely one gets flagged.¹ If a healthy person has 10 tests, the chance that all 10 come back in range is only about 60%.¹ Put another way, roughly 4 in 10 healthy people would see at least one flag.

Many everyday things can nudge a result. These include what you ate, how much water you drank, a medicine, exercise, or a menstrual period.⁸ The reverse is also true. A normal result does not guarantee good health, especially if you have symptoms.²

Here is what that means for you. A flag on your report is a reason for a conversation, not a reason to panic. The useful question is what the test was meant to answer.

Which Blood Tests Are Most Common, and What Does Each Show?

Complete blood count (CBC). This test counts the cells in your blood. Red blood cells and hemoglobin carry oxygen.⁸ White blood cells fight infection.⁸ Platelets help your blood clot.⁸ Red cell or hemoglobin levels outside the range may be a sign of anemia, too little iron, or dehydration.⁸ A high white cell count may be a sign of an infection or a reaction to a medicine.⁸

Metabolic panel (CMP or BMP). This is a group of chemistry tests run on one blood sample. The comprehensive version (CMP) measures 14 substances.⁹ They include blood sugar (glucose), calcium, and electrolytes like sodium and potassium, which help balance your body's fluids.⁹ Creatinine and BUN are waste products your kidneys filter out, so they help show how your kidneys are working.⁹ The CMP also includes liver tests.⁹ The basic version (BMP) includes 8 of the 14 tests and leaves out the liver tests.⁹

Cholesterol (lipid panel). This test measures LDL, the "bad" cholesterol that builds up in arteries.³ It also measures HDL, total cholesterol, and triglycerides, a type of fat.³ The number alone does not decide treatment. For adults 40 to 75, doctors use it to estimate your 10-year risk of a heart attack or stroke.¹⁰ They combine it with risk factors like diabetes, high blood pressure, and smoking.¹⁰ That risk decides whether a cholesterol-lowering medicine (a statin) is recommended.¹⁰

Hemoglobin A1C. This test shows your average blood sugar over the past two to three months.¹¹ An A1C below 5.7% is normal, 5.7% to 6.4% is prediabetes, and 6.5% or higher is in the diabetes range.¹¹ A high result is usually confirmed with a second test before a diagnosis.¹¹ Anemia, kidney failure, and liver disease can make A1C less accurate.¹¹ Adults 35 to 70 with overweight or obesity should be screened for diabetes.⁴ After a normal result, retesting about every 3 years may be reasonable.⁴

Thyroid test (TSH). TSH, or thyroid-stimulating hormone, is the main blood test for thyroid problems.¹² For adults without symptoms who are not pregnant, the US Preventive Services Task Force (USPSTF) found too little evidence to say whether routine screening helps.¹² Even the upper limit of normal is somewhat arbitrary. It is commonly set at 4.5 mIU/L.¹² An abnormal result should be confirmed with repeat tests over 3 to 6 months.¹²

Vitamin D. This test is popular, but the USPSTF found too little evidence to recommend for or against it in adults without symptoms.¹³ Part of the problem is that experts have not agreed on what blood level counts as enough.¹³

Hepatitis C, hepatitis B, and HIV. These screening tests are not part of a routine panel, so they have to be ordered on purpose. All adults 18 to 79 should be screened for hepatitis C.⁵ The CDC recommends that all adults 18 and older be tested for hepatitis B at least once.⁶ As many as two thirds of people living with hepatitis B might not know they have it.⁶ Everyone 15 to 65 should be screened for HIV, and people at higher risk may need repeat tests.⁷

What Does This Mean for You?

Before the draw, ask whether you need to fast. A CBC usually needs no special preparation.⁸ A cholesterol test or metabolic panel may need several hours without food, and your doctor will tell you.³ ⁹

When results come back, compare each one with the range printed on your own report. Labs use different testing methods, so their ranges differ.²

Then look at the whole picture, not a single flag. A result that is slightly high or low often moves back toward normal when it is repeated.¹ Your own past results also matter. For some tests, a change from your usual number says more than where you sit in the population range.¹

WHAT YOUR DOCTOR IS LOOKING FOR

• Why the test was ordered. The same number means different things in a screening test for someone who feels well and in a test for someone with symptoms.

• How far off, and in which direction. A result just past the line is read very differently from one far outside it.

• Your trend. For some tests, your earlier results are a better comparison than the population range.¹

• Whether related results agree. Several connected results moving together tell a stronger story than one number on its own.

• Everyday explanations first. Not drinking enough water, a new medicine, a recent workout, or a menstrual period can shift a result.⁸

QUESTIONS WORTH ASKING

• "What is each of these tests looking for, and what would change if a result is off?"

• "Do I need to fast for any of them?"

• "This result is flagged. Is it far enough out of range to matter, or should we just repeat it?"

• "How does this compare with my past results?"

• "Based on my age and risk, am I due for a hepatitis C, hepatitis B, HIV, or diabetes test?"