Pre-analytical errors in a dehydrated infant's blood samples
An 8-month-old boy with acute diarrhoea and some dehydration is admitted to a PHC. A peripheral IV line is running in his left cubital fossa. The medical officer orders a complete blood count and serum electrolytes. A lab attendant collects blood from the left arm just above the cannula site, fills a 2 mL EDTA tube only up to the 0.5 mL mark, and allows two drops to fall into a plain tube for electrolytes. The samples are left on a table near the window at 34°C and reach the district laboratory at 4 pm, six hours after collection. The CBC report shows Hb 7.2 g/dL, MCV 112 fL, MCHC 28 g/dL and platelets 90,000/µL. The serum potassium is 6.8 mmol/L, but the child is alert, passing urine, and his ECG is normal. The ward asks whether to start potassium-lowering treatment.
1. Identify the pre-analytical errors in this case and explain how each one can distort the results.
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Pre-analytical errors:
- Sampling above a running IV line - the blood is diluted by IV fluid and may be contaminated with the electrolyte solution. This can falsely lower haemoglobin and cell counts and can falsely raise or lower potassium depending on the fluid.
- Underfilled EDTA tube - a 2 mL tube filled to 0.5 mL has excess EDTA relative to blood. EDTA chelates calcium and magnesium, dilutes the sample, and alters red cell indices; poor mixing also causes microclots that falsely lower platelet counts.
- Delayed transport at 34°C - cells continue to metabolise glucose and leak potassium. Serum potassium rises falsely and glucose falls. The plain tube may also haemolyse, further raising potassium.
The Hb 7.2, MCV 112, MCHC 28 and platelets 90,000 are all unreliable. The potassium 6.8 is likely pseudohyperkalaemia because the child is clinically well, passing urine and has a normal ECG.
2. Should the ward start potassium-lowering treatment on the basis of this report? How should blood be recollected and handled?
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Do not start potassium-lowering treatment on this result alone. Treat the child, not the number: check the ECG, urine output, hydration and repeat potassium. A normal ECG and good urine output make true hyperkalaemia less likely, but the child should still be reviewed by the medical officer.
Recollection:
- Use the arm opposite the IV line, or a distal vein below the line after briefly stopping the infusion if clinically safe.
- Collect into the correct tubes: EDTA filled to the marked line and mixed gently; a heparinised or plain tube for electrolytes as per laboratory protocol.
- Label at the bedside with the time of collection.
- Transport immediately in a cool box; separate serum or plasma within 1–2 hours if possible.
- Repeat CBC and electrolytes and do not report the first results as final without a comment.
3. What changes should the PHC make to its standard operating procedure to prevent a repetition of this error?
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SOP changes:
- Write a phlebotomy SOP covering patient identification, avoiding the IV arm, correct order of draw and fill volume, and immediate mixing.
- Train lab attendants on pre-analytical errors and sample rejection.
- Maintain a sample rejection register and a rule that underfilled, clotted or delayed samples are rejected and recollected.
- Use a transport box with ice packs and a time log.
- Require the ward to inform the laboratory about IV lines and fluids.
- Audit rejected samples monthly and discuss the findings with staff.
The case should be discussed with the medical officer so that the child is not treated for a laboratory artefact. The laboratory should also issue a comment on the report explaining the likely dilution and delay, so that clinicians do not act on unreliable values.