The Diluted Truth: Discrepant Creatinine in a Trauma Patient
A 54-year-old man is admitted after a fall from height with multiple long-bone fractures. Admission labs: serum creatinine 41 µmol/L (0.46 mg/dL), urea 2.1 mmol/L, sodium 138, potassium 3.9, bicarbonate 21 mmol/L. He is started on IV fluids. On day 3 the creatinine is reported as 58 µmol/L while the patient is oliguric (urine output 15 mL/h for 6 h) and has gained 4 kg. The chemistry analyser uses an IDMS-traceable enzymatic creatinine assay. The laboratory information system shows the day-3 sample was drawn from the same arm as a running 0.9% saline infusion at 125 mL/h, and the phlebotomist documented 'line draw, 5 mL discarded' but the infusion was not paused. The clinician is reassured by the 'normal' creatinine.
1. Explain the discrepancy between the clinical picture and the reported creatinine, and state the most likely pre-analytical mechanism.
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Clinical picture: oliguria (15 mL/h) plus 4 kg weight gain indicates evolving acute kidney injury with fluid overload, which should raise creatinine. A creatinine of 58 µmol/L is implausibly low.
Mechanism: the sample was drawn downstream of a running saline infusion. Even with 5 mL discarded, the dead space of the cannula and tubing plus ongoing flow can dilute the specimen. Saline contains no creatinine, so the measured concentration falls in proportion to the dilution factor. A modest dilution (e.g., 20–40%) can mask a rising creatinine.
Key teaching point: a 'reassuring' result that contradicts the clinical trajectory should be treated as a pre-analytical problem until proven otherwise. The discard volume must exceed the line dead space, and ideally the infusion should be paused for 1–2 minutes before sampling from a distal site. Repeat from a fresh venepuncture without a line.
2. If the true (undiluted) creatinine were 120 µmol/L and the reported value 58 µmol/L, estimate the dilution factor. What else should be checked to confirm the artefact?
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Dilution estimate: reported/true = 58/120 ≈ 0.48, i.e. the specimen is roughly 52% diluted (about equal parts blood and saline).
Confirmatory checks:
- Re-collect by clean venepuncture from the contralateral arm and compare.
- Look for parallel dilution of other analytes that are absent from saline: sodium should trend toward 154 mmol/L (saline sodium) if dilution occurred, but because both plasma and saline are near-isotonic for sodium, sodium is a poor dilution marker. Better markers are total protein, albumin, and potassium - all should be proportionally lowered by saline dilution. A fall in total protein alongside the low creatinine supports dilution.
- Review the LIS for a pattern of low results on line draws from the same ward.
Action: flag the result, inform the clinician, and document the pre-analytical error. Do not act on the diluted value.
3. Draft the corrective and preventive actions you would institute in the phlebotomy SOP.
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Corrective (immediate):
- Reject/flag the affected result; repeat from a fresh site.
- Notify the requesting clinician with an amended report.
- Record a non-conformance in the QMS with root-cause analysis.
Preventive (SOP change):
- Stop or pause the infusion for at least 1–2 minutes before sampling; where impossible, sample from the opposite arm.
- Discard volume must be calculated from the line dead space (cannula + connector + tubing), typically 2–3× the dead space, not a fixed 5 mL.
- Never draw chemistry samples from a line running crystalloid without documenting the line, fluid, and rate on the request form.
- Train and competency-assess all phlebotomists; audit line-draw practice quarterly.
- Add a comment to the LIS when a sample is flagged as a line draw so the authorising scientist reviews plausibility against the clinical picture.