Blood sampling is the process of drawing a small amount of blood from your body so it can be analyzed in a laboratory. It’s one of the most common medical procedures in the world, used to check everything from blood sugar and cholesterol to infection markers and organ function. The method, equipment, and preparation involved depend on what your doctor is looking for.
How Blood Is Collected
There are three main ways blood is collected, and each serves a different purpose.
Venipuncture is the most common method. A needle is inserted into a vein, usually in the crook of your elbow or the back of your hand, and blood flows into one or more collection tubes. This is what most people picture when they think of “getting blood drawn,” and it’s used for the vast majority of routine lab work.
Capillary sampling uses a quick prick to the fingertip (or the heel in infants under six months) to collect a small drop of blood. It’s often used for point-of-care tests like blood sugar checks, or when a full venipuncture is difficult, such as in very young children or adults with hard-to-find veins. Capillary samples aren’t suitable for all tests, though. Blood cultures and most clotting tests require a venous draw.
Arterial sampling draws blood from an artery rather than a vein, typically from the wrist. It’s less common and primarily used to measure oxygen and carbon dioxide levels in the blood. Arterial draws tend to be more uncomfortable than venous ones because arteries sit deeper and have more nerve endings nearby.
What Blood Tests Actually Measure
A single blood sample can be split across several tubes and tested for dozens of different things. Three of the most frequently ordered panels give a broad picture of your health.
A complete blood count (CBC) measures your red blood cells, white blood cells, and platelets. Red blood cell counts that are too high or too low can signal dehydration, anemia, or bleeding. White blood cell abnormalities may point to infection, immune disorders, or blood cancers. Platelet levels outside the normal range can indicate clotting or bleeding problems. The CBC also checks hemoglobin, the protein that carries oxygen in your blood, and hematocrit, which reflects how much of your blood volume is made up of red blood cells.
A basic metabolic panel (BMP) measures blood sugar, calcium, electrolytes, and markers of kidney function. It’s a quick snapshot of your body’s chemical balance and is commonly ordered during routine checkups or before surgery.
A lipid panel measures total cholesterol, LDL (“bad”) cholesterol, HDL (“good”) cholesterol, and triglycerides. Abnormal levels in any of these increase your risk of heart disease, so this panel is a staple of cardiovascular screening.
Needles and Collection Tubes
The needle your phlebotomist chooses depends on the size and condition of your veins. Gauge numbers work counterintuitively: a lower number means a thicker needle. A 21-gauge needle is the standard for adults with healthy veins and allows blood to flow quickly into vacuum tubes. A 22-gauge needle is slightly thinner and better for smaller or more fragile veins, common in older adults. For children, geriatric patients, or anyone with veins that tend to roll, a 23-gauge butterfly needle is often used. It’s attached to flexible tubing that gives the phlebotomist more control, though the narrower opening slows blood flow and increases the risk of damaging the sample if drawn too fast.
The colored stoppers on blood collection tubes aren’t decorative. Each color indicates a different chemical additive inside that prepares the blood for specific tests. Lavender-topped tubes contain an anticoagulant that keeps blood from clotting and are used for CBCs. Light blue tubes contain sodium citrate for clotting studies. Gold-topped tubes have a gel that separates serum from cells after the blood clots, useful for chemistry panels. Red-topped tubes have no additive at all. A single blood draw often fills several different colored tubes, each destined for a different analysis.
Why Fasting Matters for Some Tests
You’ll typically be asked to fast for 8 to 12 hours before certain blood tests. The most common ones requiring fasting are blood glucose tests, lipid panels, and basic metabolic panels. Liver function and kidney function tests sometimes require fasting as well. Eating or drinking anything besides water during the fasting window can temporarily change your blood sugar, triglycerides, and other values, making results unreliable. If you accidentally eat before a fasting test, mention it to the person drawing your blood so the lab can account for it or your doctor can reschedule.
Common Side Effects
Blood draws are safe, but minor complications do happen. In a study of over 4,000 venipunctures, about 2% resulted in a hematoma (a bruise caused by blood leaking under the skin at the puncture site). Hematomas were more frequent in women and largely preventable by holding firm pressure on the site for three to five minutes after the needle is removed. Most people let up too early, which is the main reason bruising occurs.
Feeling lightheaded or faint is also relatively common. In the same study, about 2.6% of patients experienced sweating and near-fainting, while 0.6% fully fainted. This vasovagal response, triggered by your nervous system reacting to the sight of blood or the sensation of the needle, occurred almost exclusively in men. Occasionally the fainting episode can include brief muscle jerking that looks alarming but isn’t a seizure and doesn’t indicate a neurological problem. If you have a history of fainting during blood draws, let the phlebotomist know so you can lie down during the procedure.
What Can Go Wrong With the Sample
Even after blood is successfully drawn, the sample can be compromised before it reaches the lab. The most common problem is hemolysis, where red blood cells break apart and release their contents into the surrounding fluid. This interferes with test results and often means the draw has to be repeated. According to CDC guidelines, samples with elevated hemoglobin from broken cells can cause unreliable readings in many common tests.
Several things help prevent hemolysis: using the right needle size (20 to 22 gauge), letting the disinfectant on your skin dry completely before the needle goes in, filling each tube to its correct volume, and warming the puncture site beforehand to improve blood flow. Drawing blood too forcefully through a very thin needle is one of the most frequent causes, which is why butterfly needles, while gentler for the patient, require extra care from the person collecting the sample.
Newer Collection Methods
Several devices now make it possible to collect small blood samples with far less pain than a traditional needle stick. These use tiny microneedles, often around 1 millimeter long, that only penetrate the outermost layers of skin and avoid the deeper nerve-rich tissue that makes venipuncture uncomfortable. Some of these devices have already received FDA clearance and are classified as standard medical devices.
One example, the TAP device from YourBio Health, uses an array of solid microneedles that penetrate just 1 mm into the skin, compared to 3 to 10 mm for a standard venipuncture needle. Another, the Tasso Mini, combines a short lancet with a built-in vacuum to collect capillary blood nearly painlessly. Both are designed for home use, which opens the door to remote health monitoring and clinical trials where participants can mail samples from home rather than visiting a lab. Clinical trials are also underway exploring microneedle patches that can sample fluid just below the skin’s surface to monitor conditions like chronic pain and skin diseases, though these technologies are still in earlier stages of development.