Specimen Processing and Chain of Custody

A Result Is Only As Good As the Sample

A lab value can only guide care if the specimen belongs to the correct patient, was drawn at the correct time, and was handled correctly. Any one failure makes the number unsafe to act on. In hemodialysis, common specimens include monthly labs, adequacy (pre/post BUN) samples, blood cultures, iron and PTH studies, and ordered drug levels.

Start with identification. Verify two patient identifiers (for example, full name plus date of birth) using the facility-approved method before you draw. Dialysis units have patients with similar names who sit in the same chairs weekly, so identity confirmation is not a formality.

Label tubes at the chairside, at the moment of collection - not at the nurses' station afterward. Labeling later from memory is a leading cause of wrong-patient results, because interruptions, similar names, and multiple draws blur together.

Matching the right test to the right tube is part of this too. The lab requisition names the test; the tube color and additive must match it, and the volume must be right. A potassium ordered but drawn in the wrong tube, or a coagulation study underfilled, will be rejected - so verify the order, tube, and patient together before you ever advance the needle or open the sampling port.

Timing and the Post-BUN Technique

Timing is where adequacy labs are won or lost.

• The predialysis BUN is drawn at the start of treatment, before any heparin or saline enters the sample. Drawing through a line already flushed with saline dilutes the specimen and falsely lowers BUN.
• The postdialysis BUN must reflect true cleared blood, not blood that just passed the dialyzer or that is recirculating in the access. The standard method is the slow-flow / stop-pump technique:

Skipping the slow-flow step means the post sample is diluted by dialysate-cleared blood, BUN reads falsely low, and URR/Kt/V look falsely good. That can hide under-dialysis. Recirculation in a poorly functioning access does the same thing - cleared blood re-enters the arterial needle.

Processing, Order of Draw, and Infection Control

Processing follows the lab manual and facility procedure. Typical steps:

• Order of draw to avoid additive cross-contamination (for example, blood culture/sterile tubes before additive tubes; coagulation tubes in their required position).
• Gentle inversion of additive tubes the required number of times - never shake, which causes hemolysis that ruins potassium and other chemistries.
• Fill tubes to the correct volume so the blood-to-additive ratio is right (critical for coagulation tubes).
• Chill or protect from light only when the test requires it; place specimens in the correct biohazard bag with the requisition in the outer pocket.
• Transport within the required time - delayed samples clot, hemolyze, or degrade.

Infection control still applies at every step. Wear required PPE, avoid contaminating the outside of containers, clean blood spills promptly, and drop sharps directly into approved sharps containers. Never recap needles unless a specific engineered safety device and procedure require it. These practices reflect CDC/APIC guidance and the CMS ESRD Conditions for Coverage.

Chain of Custody and Honest Error Reporting

Chain of custody means the specimen can be tracked from collection to handoff without an unexplained gap. It typically documents the collector's identity, date and time, patient identifiers, intact seals, and each transfer. For any chain-of-custody specimen, broken seals or missing links must be reported - the result may be rejected if the chain is incomplete.

When something goes wrong, the only safe path is to report and document. A wrong tube, missing label, clotted or hemolyzed sample, delayed transport, broken container, or uncertain patient identity all make the result unsafe to use. The technician records the facts and follows policy for recollection or incident reporting.

Worked example. You realize a monthly tube was labeled with the patient who sat in that chair last week, not today's patient. You do not peel and relabel it. You discard it per policy, recollect after re-verifying two identifiers, and complete an incident report. Relabeling would risk a wrong-patient result driving a real medication or dialysate change.

Common rejection reasons - and how to prevent them

• Hemolyzed: caused by shaking, a traumatic draw, or a too-small needle - mix by gentle inversion; falsely raises potassium.
• Clotted: sat too long or was under-mixed in an additive tube - invert promptly and transport on time.
• Wrong tube / wrong order: additive carryover - follow the order of draw.
• Underfilled: wrong blood-to-additive ratio - fill coagulation tubes fully.
• Mislabeled / unlabeled: the most dangerous - label at the chairside with two identifiers.

Each rejection forces a recollection, an extra needle stick and delay for the patient. Preventing rejections is part of good, patient-centered technical care, not just lab housekeeping. When in doubt about identity or integrity, stop, verify, and report rather than send a questionable sample.