4.4 Blood Sampling Technique, Specimen Handling, and Point-of-Care Testing

4.4 Blood Sampling Technique, Specimen Handling, and Point-of-Care Testing

Quick Summary: A perfect laboratory instrument cannot rescue a badly drawn specimen. The advanced technician controls the three variables that decide whether a result is real: when in the treatment the sample is taken, where in the circuit it is taken from, and how the tube is filled, mixed, labeled, and transported.

Pre-Dialysis Sampling

Monthly laboratories - urea nitrogen, creatinine, electrolytes, calcium, phosphorus, intact parathyroid hormone, albumin, hemoglobin, ferritin, and transferrin saturation - describe the patient's steady state and must be drawn before anything in the circuit alters them.

Rules for a valid pre-dialysis draw:

1. Before heparin. Heparin does not change urea, but it invalidates coagulation studies and can interfere with selected assays. The universal teaching point is that the pre-dialysis specimen is obtained prior to the heparin bolus.
2. Before saline. Any saline infusion, including the residual prime, dilutes the sample. Draw from the arterial needle or arterial line before the blood pump is started, or immediately after connection before saline is delivered.
3. From the arterial side. The arterial needle carries blood coming from the patient. A venous-side pre-draw during running dialysis returns a dialyzed specimen.
4. Discard volume. Follow facility policy for the initial discard aliquot when drawing through a line with a residual saline column.

Sampling Through a Central Venous Catheter

Catheters are filled at the end of every treatment with a lock solution - concentrated heparin (typically 1,000 to 5,000 units/mL), 4% sodium citrate, or an alternative antimicrobial lock - that occupies the internal volume printed on each hub, generally 1.3 to 3.0 mL per lumen.

The rule is absolute: aspirate the lock and discard it before drawing any sample or initiating treatment. Aspirate the labeled priming volume plus a margin, generally 3 to 6 mL, into a syringe and discard it into a designated receptacle.

Never force an occluded catheter. Excessive pressure can fracture the catheter or dislodge a thrombus. Report resistance to the nurse; declotting with a thrombolytic is a nursing or physician function.

The Post-Dialysis Blood Urea Nitrogen

The post-dialysis urea sample decides both URR and Kt/V, and it is the specimen most often drawn incorrectly. Two effects corrupt it:

• Access recirculation - dialyzed blood re-entering the arterial needle - yields a falsely low post-BUN and a falsely inflated Kt/V.
• Compartmental (solvent) rebound - urea moving out of tissue back into blood over 30 to 60 minutes after treatment ends.

KDOQI slow-flow method:

1. At the end of the prescribed treatment time, set the ultrafiltration rate to zero.
2. Place the dialysate in bypass (or turn dialysate flow off) so no further diffusive clearance occurs.
3. Reduce the blood pump to 50-100 mL/min and wait 15-20 seconds. This clears the recirculated blood from the arterial needle without allowing meaningful rebound.
4. Draw the sample from the arterial sample port - before rinse-back, before saline.
5. Stop the pump, then complete rinse-back and termination normally.

Drawing after saline rinse-back, or drawing at full blood flow with dialysate running, is the classic error. A pair of laboratories showing an implausible jump in Kt/V should always raise the question of sampling technique before anyone congratulates the prescription.

Order of Draw and Tube Handling

When multiple tubes are filled from one draw, additive carryover between tubes changes results. The standard sequence:

Additional handling rules:

• Invert gently the number of times the manufacturer specifies - typically 8 to 10 for additive tubes. Shaking causes hemolysis, which falsely elevates potassium and invalidates the specimen.
• Fill citrate tubes completely. An underfilled blue-top has the wrong blood-to-anticoagulant ratio and produces falsely prolonged clotting times.
• Label at the chairside, in the patient's presence, before leaving the station, using two patient identifiers. Pre-labeling tubes at the nurses' station and labeling later are both mislabeling events waiting to happen; a mislabeled specimen is a reportable adverse occurrence.
• Transport per policy - refrigerated, on ice, or protected from light where required, and within the stated stability window.

Point-of-Care Testing and Quality Control

Advanced technicians run several point-of-care devices, and each has a quality-control obligation the CCHT-A blueprint names explicitly.

Blood glucose meters. Diabetes is the leading cause of end-stage kidney disease, and hypoglycemia during dialysis is common because glucose crosses the membrane. Obtain a glucose whenever a patient becomes diaphoretic, tremulous, confused, or unexpectedly hypotensive, and whenever protocol directs. Controls at two levels must be run at the interval facility policy specifies - typically each day of use and with each new lot or new operator - and results logged. A meter reading outside the control range is removed from service; it is never used with a mental correction.

Reagent test strips and colorimeters for total chlorine, water hardness, and residual germicide follow the same discipline: strips within the printed expiration date, stored capped and dry, timed exactly as the manufacturer instructs, read against the correct scale, and documented. An expired or moisture-damaged strip can read a safe 0.00 ppm while lethal chloramine passes into the dialysate.

Hematocrit and hemoglobin devices used at the chairside require the same lot-specific controls and documented operator competency.

Every point-of-care result belongs in the treatment record with the value, the time, the device identifier, and the action taken. A glucose of 47 mg/dL that never reaches the nurse and never reaches the chart is, from a survey standpoint, a result that did not happen.