Lab Results

What Happens During a Blood Test? Why Doctors Order Them and What Each Tube Means

By Health 656 Editorial Team 2026-03-29 8 min read

Blood test tubes in a medical laboratory

You sit in the chair, a rubber band tightens around your arm, and a needle draws several tubes of your blood — each a different color. You have no idea why there are multiple tubes, what each one tests, or what the results will tell your doctor. Blood tests are the most commonly ordered medical tests in the world — roughly 7 billion blood tests are performed annually in the United States alone. Yet most patients leave the lab with no understanding of what was tested or why. When results arrive in your patient portal, you see numbers, reference ranges, and flags that seem alarming but lack context. This guide explains what happens from the moment blood leaves your arm to the moment results reach your doctor — and what each test actually measures. See our detailed guide: Blood Test Results Decoded.

Why Different Color Tubes

Each tube color contains a different additive designed for specific tests. Purple/lavender (EDTA): Prevents clotting so individual blood cells can be counted and measured. Used for the complete blood count (CBC) — the most commonly ordered blood test. Light blue (sodium citrate): Preserves clotting factors. Used for coagulation tests (PT/INR, PTT) to assess blood clotting function — critical for patients on blood thinners. Red or gold (no additive or clot activator): Allows blood to clot. The serum (liquid portion after clotting) is separated and used for chemistry tests, thyroid function, liver enzymes, cholesterol, and many other tests. Green (lithium heparin): Prevents clotting for plasma chemistry tests. Gray (sodium fluoride): Preserves glucose levels — used specifically for blood sugar testing.

The order in which tubes are filled matters — contamination from one tube's additive into the next can alter results. Phlebotomists follow a strict order of draw: blood cultures first, then blue, red/gold, green, purple, gray. A single blood draw of 3-4 tubes removes roughly 10-20 mL of blood — less than a tablespoon.

The Most Common Tests and What They Reveal

Complete Blood Count (CBC): Measures white blood cells (infection fighters — elevated in infection, low in immune suppression), red blood cells and hemoglobin (anemia detection — low hemoglobin means reduced oxygen carrying), platelets (clotting — low increases bleeding risk, high increases clotting risk), and MCV (red blood cell size — small suggests iron deficiency, large suggests B12 or folate deficiency).

Comprehensive Metabolic Panel (CMP): 14 measurements including glucose (diabetes screening), creatinine and BUN (kidney function), sodium and potassium (electrolyte balance), calcium (bone and parathyroid health), liver enzymes (ALT, AST — liver damage markers), bilirubin (liver and bile duct function), and albumin (nutrition and liver function).

Lipid Panel: Total cholesterol, LDL (bad — deposits in arteries), HDL (good — removes cholesterol), and triglycerides. Requires 9-12 hours fasting for accurate triglyceride measurement. TSH: Thyroid screening — high TSH means underactive thyroid. A1C: 3-month average blood sugar — does not require fasting. Vitamin D, ferritin, B12: Specific nutrient levels that are commonly deficient.

Inflammatory markers: CRP (C-reactive protein) measures systemic inflammation. ESR (erythrocyte sedimentation rate) is another general inflammation indicator. Elevated levels suggest infection, autoimmune disease, or chronic inflammatory conditions.

What to Do With Your Results

Request a copy of every blood test. You have a legal right to your results. Do not rely on "your doctor will call if something is abnormal" — studies show that 7 percent of abnormal test results are never communicated to patients. Track your numbers over time. A single result is a snapshot. Trends tell the story. A creatinine of 1.1 is meaningless without knowing it was 0.8 two years ago (a rising trend suggesting declining kidney function).

Understand reference ranges: The range on the lab report represents where 95 percent of healthy people fall. Being slightly outside the range does not automatically mean disease. Conversely, being within the range does not guarantee health — a ferritin of 15 is technically normal but functionally deficient. Context matters more than the flag.

Ask your doctor these questions: What specifically were you looking for? Is anything trending in a concerning direction? Do I need any follow-up testing? When should these tests be repeated? These questions transform a confusing report into actionable health information.

Common Questions

Do I need to fast for blood work?

It depends on the tests ordered. Lipid panel and fasting glucose require 9-12 hours of fasting (water is always fine). A1C, CBC, thyroid, vitamin D, ferritin, and most other tests do not require fasting. Your doctor or lab should specify. When in doubt, fast — you can always eat after the draw.

How often should I get blood work?

For healthy adults with no chronic conditions: annually as part of a routine physical. For people with diabetes, kidney disease, thyroid disease, or on chronic medications: every 3-6 months as directed by your doctor. After starting a new medication that affects liver or kidney function: 4-6 weeks after initiation.

Can stress affect blood test results?

Yes. Acute stress can elevate glucose (cortisol releases stored glucose), white blood cells (stress demargination), and blood pressure. Chronic stress can elevate inflammatory markers (CRP), cortisol, and cholesterol. For the most accurate results, try to be calm and well-rested before blood work.

Why did my doctor order so many tests?

A standard panel (CBC + CMP + lipids + TSH) involves 3-4 tubes but produces 30+ individual measurements covering blood cells, organ function, electrolytes, metabolism, and thyroid. Additional tests (A1C, vitamin D, ferritin, inflammatory markers) add specific screening based on your risk factors. More tests does not mean more problems — it means more thorough evaluation.

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