Collecting a blood sample involves choosing the right method for the test, preparing the patient and equipment, drawing blood from the correct site, and handling the specimen properly afterward. Whether drawn from a vein, a fingertip, or an artery, every step in the process exists to protect both the person giving blood and the accuracy of the results.
Venipuncture vs. Capillary Collection
Most blood samples are collected through venipuncture, which means inserting a needle into a vein, typically in the inside of the elbow or the back of the hand. This method yields enough blood for multiple lab tests at once and is the standard for routine panels like a complete blood count or metabolic panel.
Capillary collection, commonly called a fingerstick, is used when only a small volume of blood is needed. The CDC recommends using the middle finger or ring finger, lancing the palm-side surface after cleaning with an alcohol swab. Capillary samples work well for point-of-care tests like blood glucose monitoring. For infants, the heel is the preferred site instead of the fingers.
Patient Preparation Before the Draw
Some blood tests require fasting for 8 to 12 hours beforehand. The most common fasting tests include blood glucose, cholesterol panels (also called lipid panels), and the basic metabolic panel. Liver function tests and kidney function panels may also require fasting in some cases. During the fasting window, water is typically fine, but food, coffee, and juice can alter results.
Beyond fasting, proper preparation means confirming the patient’s identity, verifying which tests have been ordered, and labeling tubes at the bedside or drawing station. Mislabeled specimens are one of the most common reasons samples get rejected by the lab.
Choosing the Right Needle
Needle size matters for both patient comfort and sample quality. For most adults, a standard gauge needle works well for routine venipuncture. For older adults or anyone with smaller, more delicate veins, a 22-gauge needle is a better fit. Pediatric patients, geriatric patients, and people with veins that tend to roll benefit from a 23-gauge butterfly needle, which has flexible tubing that allows for a shallower, more controlled insertion angle.
Butterfly needles are also the go-to choice for hand veins or any draw where the vein is small and difficult to anchor. The tradeoff is that smaller needles fill tubes more slowly and can sometimes cause red blood cells to break apart if blood is pulled through too quickly, which can compromise certain test results.
The Order of Draw
When multiple tubes need to be filled from a single needle stick, they must be collected in a specific sequence. This prevents additives in one tube from contaminating the next and skewing results. The standardized order, established by the Clinical and Laboratory Standards Institute, follows this sequence:
- Blood culture bottles (always first, to minimize contamination risk)
- Light blue cap (sodium citrate, used for coagulation tests)
- Red, gold, or red-speckled cap (serum tubes, with or without clot activator and gel)
- Green cap (heparin tubes, used for chemistry tests)
- Lavender, pink, or pearl cap (EDTA tubes, used for complete blood counts)
- Gray cap (contains a preservative that stops glucose from breaking down in the sample)
One important detail: plastic serum tubes that contain a clot activator can interfere with coagulation testing. If a coagulation tube (light blue) is being drawn, only blood culture bottles or glass tubes without additives should come before it.
Step-by-Step Venipuncture
The collector puts on gloves and applies a tourniquet 3 to 4 inches above the intended puncture site, then asks the patient to make a fist to help veins become more visible and palpable. After selecting a vein (the median cubital vein in the elbow crease is the most common choice), the site is cleaned with an alcohol swab in a circular motion and allowed to air dry. Inserting the needle through wet alcohol stings and can contaminate the sample.
The needle enters the skin at roughly a 15- to 30-degree angle, bevel side up. Once blood begins flowing into the first tube, the tourniquet should be released. Keeping a tourniquet on too long causes blood to pool and concentrate in the area, a phenomenon called hemoconcentration. This artificially raises the levels of proteins, electrolytes, and other substances in the sample, leading to inaccurate results. The general recommendation is to keep tourniquet time as brief as possible.
Tubes are filled in the correct order, gently inverted several times (not shaken) to mix blood with the tube’s additives, and then the needle is withdrawn. Gauze is placed over the site with gentle pressure, and a bandage is applied once bleeding stops.
Safety and Sharps Disposal
Federal workplace safety standards require that used needles and lancets go immediately into a sharps container that is puncture-resistant, leakproof, and clearly labeled. These containers must stay upright, remain easily accessible during the procedure, and be replaced before they become overfull. Recapping needles by hand is one of the leading causes of accidental needlesticks and is prohibited under standard protocols.
Gloves are mandatory for every blood draw. Depending on the setting, additional protective equipment like gowns, masks, or eye protection may be required. After the procedure, work surfaces need to be cleaned with an appropriate disinfectant, especially if any blood was spilled during the collection. This decontamination also happens at the end of every shift in areas where blood draws routinely take place.
Handling Samples After Collection
What happens to a blood sample in the minutes after collection is just as important as the draw itself. Mishandled specimens produce unreliable results and often need to be redrawn.
Serum tubes (red or gold cap) need to sit upright for at least 30 minutes to allow the blood to clot. After clotting, they should be spun in a centrifuge within one hour, and no later than two hours after collection. For any tube where red blood cells need to be separated from serum or plasma, that separation must happen within two hours or results can shift.
Temperature matters enormously. Placing an unspun tube in the refrigerator can increase potassium levels by as much as 135%, which could trigger a false critical-value alert. Unspun serum and plasma tubes should never be refrigerated. Once tubes have been centrifuged and properly separated, storage requirements vary by test. Some specimens stay at room temperature, others need refrigeration, and some must be frozen. Frozen specimens require transferring the separated serum or plasma into a separate tube before freezing.
During transport, specimens that need to stay cold go in designated bags that signal the courier to use ice packs. Specimens requiring frozen transport travel on dry ice. Room-temperature specimens are sealed in biohazard-labeled bags and transported without additional temperature control.
Common Reasons Samples Get Rejected
Labs reject samples more often than most people realize, and a rejected sample means the patient gets stuck again. The most frequent causes include hemolysis (red blood cells breaking apart, often from using too small a needle, pulling blood too forcefully, or shaking tubes instead of gently inverting them), clots in tubes that are supposed to remain liquid, underfilled tubes that throw off the ratio of blood to additive, and incorrect labeling.
For coagulation testing in particular, the light blue citrate tube must be filled to its marked line. An underfilled citrate tube changes the blood-to-anticoagulant ratio enough to make clotting time results meaningless. If the coagulation tube is the first and only tube being drawn, some facilities require a small “discard” tube to be filled first, clearing the needle of any tissue fluid that entered during the initial puncture.