9.2 Blood Sampling, Laboratory Trends and Critical Results

Start With the Clinical Question

A blood sample is useful only when it represents the intended condition. Predialysis chemistry, postdialysis adequacy BUN, blood cultures, anticoagulation studies and drug levels require different timing and collection methods. The nurse verifies the order, patient identity, sample site, container and handling instructions before collection. A tube correctly labeled with the patient's name can still be clinically misleading if collected from the wrong location or at the wrong time.

Collect using aseptic technique and the approved circuit or venipuncture procedure. Avoid access sites reserved for dialysis except under the appropriate plan. Trace ports and connections rather than guessing from color. Follow the laboratory's order of draw and mixing instructions. Label at the bedside using the required identifiers and actual collection time; a pre-labeled tube taken to another patient creates a serious identity risk.

Predialysis and Postdialysis BUN

Adequacy samples should reflect the pre- and post-treatment concentrations specified by the kinetic method. Obtain the predialysis sample before saline or medication dilution as required by the protocol. For post-BUN, use the standardized arterial sampling method. A sample from the dialyzer venous outlet can be artificially low because it contains recently cleared blood, overstating delivered adequacy.

A commonly used slow-flow method reduces blood flow to 100 mL/min for approximately fifteen seconds before arterial collection, with the exact clamps and sampling site defined by facility procedure. It reduces access-recirculation effects but does not eliminate all cardiopulmonary or tissue rebound. Do not substitute a convenient sample after a large rinseback. Document timing, delivered time and interrupted treatment so a surprising result can be investigated.

For pre-BUN 60 mg/dL and post-BUN 18 mg/dL, URR is (60−18)/60 ×100 = 70%. If the post sample is accidentally diluted to 12, the apparent URR becomes 80%, although treatment has not improved. Rechecking the collection method can therefore be as important as reviewing the dialyzer.

Chemistry and Hemolysis

A hemolyzed specimen may have falsely high potassium. Distinguish specimen artifact from hemolysis in the patient, which can produce dangerous potassium release and requires emergency action. The laboratory flag, clinical condition, ECG, circuit appearance and repeat testing all inform assessment. Do not dismiss a critical potassium as artifact simply because hemolysis is common in blood collection. Conversely, an isolated compromised specimen should not generate an unexamined permanent bath change.

Calcium results need their units and type: total calcium in mg/dL differs from ionized calcium in mmol/L. Albumin-adjusted estimates can be inaccurate in dialysis, especially with acid-base changes. For citrate monitoring, convert total and ionized calcium to matching units before forming a ratio. Sodium, bicarbonate and phosphate also depend on timing, residual function, nutrition and actual delivered therapy.

Coagulation and Medication Levels

Do not compare a postdialysis drug level with a predialysis target without the pharmacy plan. Rebound and redistribution may make immediate post-treatment levels misleading. A heparinized line can falsely prolong coagulation results if collected improperly. Record the source and follow the required discard or alternative collection process rather than assuming every catheter sample is equivalent.

Infection Samples and Critical Communication

When bloodstream infection is suspected, obtain ordered cultures promptly using the specified sites and technique, preferably before antibiotics when this does not delay essential treatment. Note fever, chills, access findings and timing relative to dialysis. Negative cultures do not by themselves establish a pyrogenic water reaction, especially if antibiotics were given or collection was limited. The clinician evaluates the whole course while technical staff assess any suspected shared exposure.

Critical results require patient assessment and communication within policy-defined urgency. State the value, units, sample timing, symptoms and any protective action. Read back orders when required and confirm the responsible clinician has received the result. A message left without confirmation is not always sufficient for an emergency. Document the response and arrange repeat testing or transfer as directed.

Trend-Based Review

Compare results across sessions using consistent collection conditions. A falling hemoglobin may reflect bleeding, dilution, inflammation or reduced erythropoiesis; an isolated value is not an instruction to double ESA. A decrease in delivered Kt/V may reflect shorter time, low flow, access problems or sample error. Investigate the mechanism before changing treatment. The nurse's laboratory skill includes obtaining a trustworthy specimen and connecting it with an appropriate clinical decision, not merely memorizing reference ranges.

Sources: KDOQI sampling methods, KDIGO anemia guidance.