Clinical Skills · Specimen Collection and Lab Testing
Blood Sampling
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In 30 seconds
Blood sampling is the skill of obtaining a small amount of blood for laboratory testing. The two most common methods are Venipuncture Puncture of a vein with a needle to obtain blood Full entry → — inserting a needle into a vein to fill collection tubes — and Capillary sampling Collecting small blood droplets from a fingertip or heel stick Full entry → — a small lancet stick of a fingertip or heel to collect tiny droplets of blood. Blood is drawn for many reasons: to diagnose illness, to monitor chronic conditions, to check the effects of treatment, and to screen for problems before they cause symptoms.
Who performs blood sampling varies widely. In some settings a dedicated phlebotomist draws most samples; in others, nurses draw blood as part of their daily work, and in many places nurses perform point-of-care capillary tests (such as blood glucose checks) regularly. The scope of what a nurse may do depends on jurisdiction, facility policy, and education — always practice within your own scope.
The most important idea in this topic: most laboratory errors happen before the sample ever reaches the analyzer. Collecting from the wrong patient, filling the wrong tube, letting a sample hemolyze, or delaying transport can all produce results that are wrong, misleading, or unusable — no matter how good the laboratory is. The nurse's careful technique is the first and best quality control.
Why this matters
- Results drive decisions. Clinicians treat based on laboratory values, so an inaccurate result can lead to undertreatment, overtreatment, or harmful treatment.
- Patient identification is a safety event, not a formality. Drawing the wrong patient's blood is a serious, reportable error; verifying identity before the needle touches skin is non-negotiable.
- Preanalytical errors are the biggest source of lab error. Hemolysis Rupture of red blood cells, releasing their contents into the sample Full entry →, underfilled tubes, clotted anticoagulated samples, and delayed transport are common and mostly preventable.
- Blood is a body fluid. Standard precautions, careful handling, and safe sharps disposal protect the nurse, the patient, and everyone downstream.
- It is a high-frequency nursing skill. Expect it in clinical rotations, skills check-offs, and exam questions about Order of draw The fixed sequence for filling collection tubes Full entry →, patient ID, and error prevention.
The college version
Core Concepts
Venipuncture basics
Venipuncture means "puncturing a vein." The usual sites are the veins of the antecubital fossa (the bend of the elbow) — the median cubital, cephalic, and basilic veins — because they are large and relatively stable. General rules for site selection:
- Avoid limbs with an intravenous (IV) line in progress, a dialysis fistula or graft, scars, burns, or edema, and — per facility policy — the arm on the side of a mastectomy or lymph node dissection.
- Choose a straight, visible, palpable vein; palpation (feeling) is more reliable than sight alone.
- Equipment typically includes a tourniquet, an antiseptic (alcohol or chlorhexidine per facility policy), a needle or winged "butterfly" set, collection tubes or a syringe, gauze, and a sharps container.
A general technique sequence: verify the patient's identity with two identifiers → confirm the ordered tests and any special requirements (such as fasting) → perform hand hygiene and apply gloves → apply the tourniquet → select the site → cleanse the skin and allow it to dry → anchor the vein → insert the needle bevel-up → collect the tubes in the correct order → release the tourniquet → withdraw the needle → apply firm pressure → label the tubes at the bedside → dispose of the needle immediately in a sharps container → document. Exact steps and devices differ by facility, so the procedure manual and supervised practice, not this summary, are the authority.
Order of draw and tube additives
Collection tubes contain additives — anticoagulants, clot activators, or preservatives — that prepare the blood for specific kinds of tests. Which tube is used depends on which test is ordered. Tubes must be filled in a specific order of draw so that Additive A substance inside a collection tube (anticoagulant, clot activator, preservative) Full entry → from one tube cannot contaminate the next; for example, blood culture bottles are collected first, before tubes containing additives. Tubes with additives must be gently inverted the required number of times so the blood mixes with the additive — too little mixing and the sample clots; too much agitation and the red cells may break apart (hemolyze). An underfilled tube may be rejected by the laboratory. Learn the order of draw used at your facility from its current policy and reference materials.
Capillary sampling
Capillary sampling collects small volumes of blood from a fingertip (adults and older children) or, in infants, from the heel — the sites and lancet sizes are chosen per facility policy. The site may be warmed first to increase blood flow. A small lancet makes a shallow puncture, the first drop is handled according to the test's instructions, and the blood is collected into a tiny tube or applied directly to a test strip or cartridge. Capillary sampling is commonly used for point-of-care tests (such as blood glucose) and for pediatric specimens where only a small amount is needed. Firm pressure after the stick prevents oozing.
Preventing preanalytical errors
The most common preventable errors, and how to prevent them:
- Wrong patient: verify two identifiers every time; label at the bedside.
- Hemolysis (red cells destroyed): avoid excessive tourniquet time, vigorous tube mixing, small needles, and drawing from a difficult or fragile site.
- Clotted anticoagulated sample: invert tubes promptly and completely, the number of times the manufacturer specifies.
- Underfilled or overfilled tube: fill to the tube's marked fill line.
- Wrong tube: confirm which tube the test requires before the draw.
- Improper transport: deliver promptly; some tests need ice, protection from light, or specific temperature handling — follow lab instructions.
Aftercare, complications, and safety
Apply pressure to the site until bleeding stops, then apply a bandage. Watch for a Hematoma A collection of blood under the skin from a vessel leak Full entry → (a bruise-like collection of blood under the skin) and apply pressure if one forms. Some patients feel lightheaded during or after a draw — a vasovagal response; have the patient lie down or sit with the head lowered, and do not leave the patient unattended until recovered. Report unusual bleeding, persistent pain, or numbness, which can signal a problem such as nerve involvement. Always use standard precautions: gloves, hand hygiene, and immediate disposal of sharps in a puncture-resistant container. Never recap needles.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Venipuncture | Capillary sampling | Venipuncture uses a needle in a vein and fills tubes; capillary sampling uses a lancet stick and collects tiny droplets |
| Serum | Plasma | Both are the liquid part of blood, but plasma still contains clotting factors; they are obtained from different tubes |
| Hemolyzed sample | Clotted sample | Hemolysis is broken red cells (from rough handling or a difficult draw); a clot is blood that solidified because anticoagulant didn't mix in |
| Venous sampling | Arterial sampling | Venous draws are routine blood tests; arterial samples (for blood gas analysis) are a different procedure with different sites and handling |
| A "successful" draw | A "good" draw | Getting blood is not enough — the right patient, right tube, right fill, right mixing, and prompt transport all determine quality |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Blood sampling is like taking a tiny scoop of water from a river to check what's in it. The nurse first makes sure the scoop is from the right river — that's checking your name and birthday — then takes a very small sample from a vein in your arm. The sample goes into special tubes so the lab can count cells, measure chemicals, or look for germs. Because the tests depend on the sample, the nurse is extra careful that it's yours, it's in the right tube, and it gets to the lab fast.
Worked example
Nurse Okafor needs to draw a complete blood count and a glucose test on Ms. Iyer, who is fasting. She confirms the fasting requirement, then verifies Ms. Iyer's identity using her name and date of birth and the wristband. She selects the median cubital vein in Ms. Iyer's left arm — the right arm has an IV infusing. After cleansing and letting the antiseptic dry, she anchors the vein and inserts the needle. The first tube (with anticoagulant for the blood count) fills; she inverts it gently the required number of times. The second tube fills for the glucose test. She releases the tourniquet, withdraws the needle, and holds firm pressure on the site while Ms. Iyer rests for a moment. She labels both tubes at the bedside, checks them against the wristband one more time, disposes of the needle directly into the sharps container, and sends the specimens to the lab promptly. When Ms. Iyer mentions she felt a little dizzy, Nurse Okafor keeps her lying down until the feeling passes before helping her sit up.
Key takeaways
- Two patient identifiers before every draw; label tubes at the bedside. This prevents the most serious error of all.
- Most lab errors are preanalytical — wrong tube, hemolysis, clotting, underfill, delayed transport — and are preventable with technique.
- Order of draw prevents additive cross-contamination; follow the facility's current reference.
- Tubes with additives must be gently inverted the required number of times; underfilled tubes may be rejected.
- Venipuncture ≠ capillary sampling: different sites, equipment, and typical uses.
- Aftercare: hold pressure until bleeding stops, watch for hematoma, and stay with a lightheaded patient until recovered.
- Sharps safety: never recap, dispose immediately, standard precautions always.
- Who may draw blood and which methods a nurse may use vary by jurisdiction, facility policy, and scope of practice.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why must the nurse verify the patient's identity with two identifiers before drawing blood?
Show answer
Because labeling and treating the wrong patient's blood is a serious, preventable error. The test results will be used to make treatment decisions, so the sample must unquestionably belong to that patient.
What is the purpose of the order of draw, and what happens if it is not followed?
Show answer
The order of draw prevents additive from one tube from contaminating the blood in the next tube. If it is not followed, test results can be inaccurate or the sample unusable.
Name three preanalytical errors and how each is prevented.
Show answer
(Any three): wrong patient — verify two identifiers and label at bedside; hemolysis — avoid excessive tourniquet time, vigorous mixing, and difficult draws; clotting — invert anticoagulant tubes promptly and correctly; underfilled tube — fill to the fill line; delayed transport — deliver promptly per lab instructions.
Why are tubes with additives gently inverted after filling?
Show answer
So the anticoagulant or other additive mixes evenly with the blood. Too little mixing → clots; too much agitation → hemolysis.
A patient becomes lightheaded during a blood draw. What should the nurse do?
Show answer
Stop the procedure if needed, keep the patient lying down or lower the head, monitor until the feeling passes, and do not leave the patient unattended. Apply pressure to the site and seek help if the patient does not recover promptly.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Venipuncture
- Puncture of a vein with a needle to obtain blood
- Capillary sampling
- Collecting small blood droplets from a fingertip or heel stick
- Additive
- A substance inside a collection tube (anticoagulant, clot activator, preservative)
- Order of draw
- The fixed sequence for filling collection tubes
- Hemolysis
- Rupture of red blood cells, releasing their contents into the sample
- Anticoagulant
- An additive that keeps blood from clotting
- Hematoma
- A collection of blood under the skin from a vessel leak
- Preanalytical error
- A mistake occurring before the sample is analyzed
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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