Laboratory tests help your physician assess your health, diagnose conditions, and monitor treatments. Knowing what these tests measure and how to prepare for them helps you participate actively in your care.

When your physician orders laboratory tests, you may wonder what the tests measure, why they matter, and how to prepare. Lab work provides objective information about your body's chemistry, cell counts, organ function, and disease markers that physical examination alone cannot reveal.

Thousands of different laboratory tests exist, but most patients encounter a smaller group of common tests during routine checkups, illness evaluation, or chronic disease management. Understanding these tests helps you prepare properly, ask informed questions, and work with your physician to make health decisions based on the results.

Complete Blood Count (CBC)

The complete blood count measures the cells in your blood: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help blood clot. Your physician may order a CBC during routine checkups, before surgery, or when you have symptoms like fatigue, weakness, fever, or unusual bruising.

Red blood cell measurements include hemoglobin (the oxygen-carrying protein) and hematocrit (the proportion of blood made up of red cells). Low values may indicate anemia from iron deficiency, chronic disease, or blood loss. High values may signal dehydration or certain blood disorders.

White blood cell counts help evaluate your immune system and detect infection or inflammation. The differential count breaks down white cells into types—neutrophils, lymphocytes, monocytes, eosinophils, and basophils—each serving different immune functions. Platelet counts assess your blood's clotting ability. No special preparation is typically needed for a CBC.

Basic Metabolic Panel and Comprehensive Metabolic Panel

The basic metabolic panel (BMP) measures electrolytes (sodium, potassium, chloride, bicarbonate), kidney function markers (creatinine, blood urea nitrogen), and blood glucose. The comprehensive metabolic panel (CMP) includes these plus liver function tests (albumin, total protein, liver enzymes, bilirubin). Physicians order these panels to screen for diabetes, kidney disease, liver problems, and electrolyte imbalances.

Electrolytes regulate fluid balance, nerve signals, and muscle function. Abnormal levels may result from dehydration, kidney disease, medications, or hormonal disorders. Creatinine and blood urea nitrogen reflect how well your kidneys filter waste. Glucose measurement helps diagnose and monitor diabetes.

Liver enzymes (ALT, AST, alkaline phosphatase) detect liver inflammation or damage. Bilirubin measures a yellow pigment that can accumulate when liver function declines or red blood cells break down excessively. You typically need to fast for 8-12 hours before these tests for accurate glucose and lipid measurements if ordered together.

Lipid Panel

A lipid panel measures cholesterol and triglycerides—fats in your blood that affect cardiovascular disease risk. The panel typically includes total cholesterol, LDL (low-density lipoprotein, often called “bad” cholesterol), HDL (high-density lipoprotein, “good” cholesterol), and triglycerides.

LDL cholesterol can deposit in artery walls, contributing to blockages that cause heart attacks and strokes. HDL cholesterol helps remove cholesterol from arteries. Triglycerides are a fat type that increases when you consume excess calories, especially from sugar and alcohol. High triglycerides combined with other abnormal lipid values increase cardiovascular risk.

Your physician uses lipid results along with other risk factors—age, blood pressure, diabetes, smoking, family history—to assess cardiovascular disease risk and guide treatment decisions. Some lipid tests require fasting for accurate triglyceride measurement, though newer methods may allow non-fasting samples. Ask your physician's office about preparation requirements when scheduling.

Hemoglobin A1C

Hemoglobin A1C (also called HbA1c or glycated hemoglobin) measures average blood glucose levels over the past two to three months. Unlike daily glucose checks that capture a single moment, A1C reflects longer-term glucose control by measuring the percentage of hemoglobin proteins with glucose attached.

Physicians use A1C to diagnose diabetes and prediabetes and to monitor glucose control in patients with diabetes. The test requires no fasting and can be done anytime. Results appear as a percentage: below 5.7% is generally considered normal, 5.7-6.4% indicates prediabetes, and 6.5% or higher suggests diabetes on repeat testing.

For people with diabetes, the A1C goal varies based on individual circumstances, but many physicians recommend below 7% for most adults. Your physician considers factors like age, other health conditions, and risk of low blood sugar when setting your target. Regular A1C testing—typically every three to six months for people with diabetes—helps adjust treatment plans.

Thyroid Function Tests

Thyroid function tests measure hormones that regulate metabolism, energy, body temperature, and many other functions. The most common test measures thyroid-stimulating hormone (TSH), produced by the pituitary gland to control the thyroid. Additional tests may measure the thyroid hormones T4 and T3.

High TSH with low thyroid hormone indicates hypothyroidism (underactive thyroid), causing symptoms like fatigue, weight gain, cold sensitivity, and constipation. Low TSH with high thyroid hormone indicates hyperthyroidism (overactive thyroid), causing symptoms like anxiety, weight loss, rapid heartbeat, and heat intolerance.

No special preparation is needed for thyroid tests, though some physicians prefer testing in the morning. If you take thyroid medication, ask whether to take your dose before the blood draw, as timing can affect results. Your physician may order thyroid antibody tests if autoimmune thyroid disease is suspected.

Lab Test Preparation Checklist

Confirm fasting requirements and duration when scheduling (typically 8-12 hours for metabolic and lipid panels)
Ask whether to take your regular medications before the test or bring them to take afterward
Drink water unless instructed otherwise—staying hydrated makes blood draws easier
Wear a short-sleeved shirt or loose sleeves that roll up easily
Bring your insurance card, identification, and test order from your physician
Write down questions about the tests to ask the lab staff or your physician
Ask when and how you will receive results and who will contact you to discuss them
Schedule a follow-up appointment or phone call to review results if not already planned

When to Contact a Clinician

Contact your physician's office if you do not receive test results within the timeframe they specified, typically one to two weeks. Reach out sooner if you develop new or worsening symptoms while waiting for results. If you receive results through an online patient portal without explanation, call to discuss what the numbers mean and whether any action is needed—do not assume normal results mean no follow-up is necessary, and do not attempt to interpret abnormal results on your own.

Questions to Ask at a Medical Visit

What specific tests are you ordering and why?
Do I need to fast or make other preparations before the blood draw?
Should I take my regular medications before the test?
When and how will I receive the results?
Will someone contact me to explain the results, or do I need to schedule a follow-up?
What do you expect to find, and what happens if results are abnormal?
How often will I need to repeat these tests?

Main Takeaway

Laboratory tests provide essential information that guides your medical care, from routine health screening to chronic disease management. Understanding what common tests measure, how to prepare for them, and how to follow up on results helps you partner with your physician effectively. When you know what to expect, you can ensure accurate results, ask better questions, and make informed decisions about your health based on objective data rather than guesswork.

References and Further Reading

American Association for Clinical Chemistry — Lab Tests Online patient education on common blood tests
Centers for Disease Control and Prevention — Diabetes testing and A1C information
National Heart, Lung, and Blood Institute — Cholesterol testing and lipid panel interpretation
American Thyroid Association — Thyroid function test basics for patients
National Kidney Foundation — Kidney function testing and creatinine measurement

Understanding Common Laboratory Tests: A Patient's Guide