4.1 Blood Sampling & Blood Culture Collection

Phlebotomy as Domain 1C clinical skill

Phlebotomy in infusion nursing is more than “getting blood.” Domain 1C (Phlebotomy) on the CRNI content outline (including the September 2025 outline) expects you to choose the right access route, protect specimen quality, prevent contamination, and distinguish diagnostic sampling from therapeutic blood removal. This section focuses on diagnostic sampling and blood cultures; therapeutic phlebotomy is covered next.

Exam stems often mix a patient with an existing central line, a needed culture or chemistry panel, and a tempting “just draw from the PICC” option. Correct answers weigh accuracy, infection risk, device function, and facility policy—not convenience alone.

Peripheral venipuncture vs sampling from vascular access devices

Peripheral venipuncture

Peripheral venipuncture remains the preferred default for many routine laboratory studies when veins are available and the patient can tolerate a stick. Advantages include:

• Lower risk of catheter lumen contamination or false-positive cultures from a colonized hub
• No need to interrupt continuous infusions on a multi-lumen device (when the peripheral draw is independent)
• Avoidance of discard/waste volume that may matter in infants, anemia, or frequent sampling

Technique essentials: appropriate tourniquet time (avoid prolonged stasis that can alter some analytes), correct needle or winged-set gauge for the vein and test, skin antisepsis with full dry time, stable positioning, and gentle fill of tubes. Release the tourniquet once flow is established when appropriate for the method used.

Sampling from vascular access devices (VADs)

Drawing from a peripheral IV, midline, PICC, non-tunneled CVC, tunneled catheter, or implanted port can spare veins and is sometimes necessary, but it introduces dead-space, drug residue, and contamination considerations.

Key principles:

1. Stop incompatible or interfering infusions on the lumen (or adjacent lumens as policy requires) long enough to avoid specimen contamination with IV fluids, electrolytes, or medications that alter results.
2. Flush with preservative-free 0.9% sodium chloride as indicated to clear residual infusate (follow lumen volume and policy).
3. Discard (waste) volume or use an approved push-pull (mixing) method before collecting the specimen so dead-space fluid and residual drug do not dilute or contaminate the sample.
4. Collect into the correct tubes in the proper order of draw when multiple tubes are needed.
5. Flush and restore the lumen; restart infusions only after the line is cleared and patent.
6. Document the source (peripheral vs which device/lumen), method, and any deviations.

Waste volume, push-pull, and dead-space

Dead-space is the internal volume of the catheter, extension set, and needleless connector that holds residual fluid. If you sample without clearing it, you measure diluted drug, TPN residue, or saline—not true circulating blood values.

• Waste method: Aspirate and discard a volume at least equal to the system dead-space plus a safety margin defined by policy (often several times the calculated dead-space for central lines; always follow manufacturer and institutional standards). Never return discard blood to the patient unless using a closed blood-conservation system designed and approved for that purpose.
• Push-pull (mixing) method: When policy allows, blood is gently aspirated and reinfused several times to mix lumen contents with circulating blood, then the specimen is drawn without a large discard—useful for blood conservation. Use only if validated by facility protocol for that device and analyte set.
• Blood conservation systems: Closed systems that return discard volume reduce iatrogenic anemia; use per product instructions and infection-prevention standards.

Exam trap: Drawing “a little blood for labs” from a TPN lumen without waste or flush—results will be invalid and the lumen is put at infection risk from extra manipulation.

When NOT to draw from certain devices

Do not treat every lumen as a free lab access. Situations that commonly prohibit or restrict sampling:

• Dedicated lines where policy forbids interruption (e.g., continuous vasoactive infusions without another lumen or peripheral access for labs)
• Hemodialysis catheters used only by dialysis-trained staff under strict protocols—routine ward draws are usually prohibited
• Lines with known or suspected malfunction, occlusion, or infection signs when drawing would delay management or worsen risk
• Certain implanted ports or devices if access technique, non-coring needle size, or power/injection restrictions make sampling unsafe or noncompliant with labeling
• Peripheral catheters that are newly infiltrated, phlebitic, or not flushing freely—do not force aspiration
• Cultures through existing lines when line infection is not the question—prefer a fresh peripheral stick for routine bacteremia workups (see blood cultures below)

If a draw from a line is clinically required, choose the most appropriate lumen, stop interfering infusions, and document rationale.

Hemolysis and contamination reduction

Hemolysis falsely elevates potassium and LDH, lowers haptoglobin interpretation reliability, and can invalidate many tests. Prevention strategies:

• Avoid excessively small-gauge needles for large vacuum tubes when a larger gauge is safe for the vein
• Do not pull the syringe plunger with high force; allow vacuum tubes to fill by vacuum without forcing
• Fill tubes gently; mix by inversion—do not shake
• Avoid drawing from sites distal to IV infusions or through partially occluded catheters
• Transfer blood correctly if using a syringe (use transfer device; do not needle through the stopper aggressively when alternatives exist)
• Minimize tourniquet time and fist clenching for potassium-sensitive draws when possible

Contamination of chemistry or coag specimens can come from IV fluids (e.g., spurious hypernatremia or hyperglycemia from D5NS), while microbial contamination ruins culture interpretation. For cultures, skin and hub flora are the classic false-positive sources.

Order of draw principles

When multiple tube types are collected, follow the facility’s order of draw (aligned with current CLSI-based sequences) to limit additive carryover. A common conceptual order (verify current local protocol) prioritizes:

1. Blood culture bottles first (when cultures are ordered with other labs)
2. Coagulation tubes (citrate) after cultures, with attention to correct fill volume
3. Serum tubes with or without clot activator/gel
4. Heparin tubes
5. EDTA tubes
6. Glycolytic inhibitor tubes last among common chemistry sets

Wrong order can cause EDTA contamination of calcium/potassium results or citrate underfill that invalidates coagulation studies. Always fill citrate tubes to the marked line.

Blood culture collection

Blood cultures diagnose bacteremia/fungemia and support CRBSI evaluation. Quality hinges on antisepsis, volume, number of sets, and source selection.

Skin antisepsis and aseptic technique

• Perform hand hygiene; assemble supplies before skin prep.
• Scrub the venipuncture site with the facility-approved antiseptic (often >0.5% chlorhexidine in alcohol or alcohol followed by iodine products per protocol); allow complete dry time—do not wipe dry.
• Do not re-palpate the cleansed site with a non-sterile finger.
• Disinfect bottle tops with alcohol and allow to dry before inoculation.
• Use sterile or clean non-touch technique for needle and bottle access as required.

Volume adequacy and pairs of bottles

Volume is the single most important factor for adult culture sensitivity. Underfilled bottles miss organisms. Follow manufacturer adult and pediatric volume ranges (adults often need ~8–10 mL per bottle when product allows; pediatrics are weight-based—know that “a few drops” is usually inadequate in adults).

Typical sets include aerobic and anaerobic bottles (a “pair”). Obtain two sets from separate sites or sequential draws per sepsis protocol to improve yield and help interpret contaminants. Timing: draw before antibiotics when possible; if antibiotics already started, still obtain cultures promptly and document timing.

Peripheral vs catheter draws for cultures

• When line infection is not suspected: Prefer peripheral venipuncture for blood cultures. Drawing through an existing line increases false-positive risk from hub/lumen colonization and complicates interpretation.
• When CRBSI is suspected: Obtain paired cultures—at least one set from a peripheral venipuncture and concurrent sets from the catheter lumen(s)—using identical volumes when possible. Document exact sources and times. Differential time to positivity (DTP) or quantitative methods may be used by the lab/clinician to support catheter-related infection; the nurse’s job is correct paired collection and labeling.
• If a culture must be drawn from a line for other reasons, document the source clearly so results are not misread as peripheral bacteremia proof.

Exam trap: Drawing all cultures only from the central line “because the patient is a hard stick,” then treating any positive as definitive CRBSI without peripheral comparison when CRBSI workup was the goal.

Timing relative to antibiotics

Whenever feasible, collect cultures before the first antibiotic dose. If the patient is already on therapy, draw cultures without delaying needed antimicrobials further when sepsis is severe—document antibiotic timing so clinicians interpret negative cultures in context. Do not skip cultures solely because antibiotics were given hours earlier; still obtain them as ordered.

Lab labeling, identification, and pre-analytic safety

• Identify the patient with two identifiers before collection.
• Label at the bedside immediately after draw: patient name/ID, date, time, collector initials, and specimen source (peripheral right arm, PICC proximal lumen, port, etc.).
• Never pre-label empty tubes away from the patient in a way that risks mix-ups.
• Note difficult draws, prolonged tourniquet, IV site proximity, or hemolyzed appearance when relevant.
• Transport per lab requirements (ice for some analytes, protect from light, timing for lactate or drug levels).

Putting it together: exam-style reasoning

Scenario: A stable patient needs morning chemistries and has a double-lumen PICC infusing antibiotics in one lumen. Best approach is often peripheral venipuncture if easy; if drawing from the PICC is necessary, stop the infusion as policy requires, flush, waste or push-pull per protocol from the appropriate lumen, collect, flush, and restart—never sample from a running medication without clearing dead-space.

Scenario: Febrile patient with a CVC, concern for CRBSI. Collect peripheral culture set plus catheter lumen sets, full volumes, meticulous antisepsis, label sources, then start antibiotics after cultures when the clinical picture allows.

Master peripheral-vs-device decisions, waste/dead-space logic, culture technique, and labeling, and you will handle Domain 1C diagnostic sampling items with confidence before moving to therapeutic phlebotomy.