Central line blood draws: pausing infusions, discard volume and contaminated samples

Why the sampling route changes the result

A central venous catheter offers painless access for patients who need frequent tests, but every lumen holds fluid that is not blood. Between uses it contains flush solution, and an active lumen carries whatever is infusing. If that fluid reaches the tube, the laboratory measures a mixture of blood and infusate rather than the patient's circulation.

Laboratory groups discourage routine catheter sampling because it raises the risk of haemolysis and of admixture with intravenous fluid or flush. When the line is the only realistic option, the nurse's job is to make the sample represent the patient. That depends on pausing infusions, selecting the lumen, discarding the dead-space blood and labelling the sample accurately.

Critical steps before the first tube

Check the order, the patient's identity and which tests are needed, then review what runs through each lumen. Pause infusions in all lumens for the time your policy specifies, clamping rather than relying on a pump pause alone. Some infusions, such as vasoactive drugs, cannot safely be stopped; follow the local rule for drawing from a dedicated lumen.

Perform hand hygiene, wear gloves and scrub the needleless connector with an approved antiseptic, letting it dry. Withdraw the discard volume set by your protocol and set it aside so it cannot be mistaken for a specimen. A study in critically ill adults found a smaller discard performed as well as the traditional larger one, which matters for patients losing blood to repeated testing.

Collect, flush and restart in the right order

Attach the collection device and fill tubes in the laboratory's order of draw, mixing additive tubes gently. Withdraw slowly: forceful aspiration through a narrow lumen can haemolyse cells and pull fluid from neighbouring lumens. If blood cultures are ordered, follow the culture protocol for line and peripheral sets, because results are interpreted by source.

Flush the lumen with the volume and solution your policy specifies, using a pulsatile technique, then restart each infusion at its prescribed rate and confirm the pump settings. Label tubes at the bedside with the time and the line used. Drawing from a central line is a sterile entry into the bloodstream, so a dropped cap or touched hub means starting again.

Recognising a contaminated or misleading sample

Dextrose contamination is the classic error. Glucose solutions are far more concentrated than blood, so a very small volume of infusate can push the measured glucose to a startling level. One laboratory service traced contamination to pump pauses instead of clamps, unclamped lumens, too little waiting time and aggressive aspiration. A falsely high glucose can prompt insulin and cause dangerous hypoglycaemia.

Other clues include a sudden electrolyte pattern matching the infusion, unexplained dilution of haemoglobin, or a result that contradicts the patient's condition. Drug levels deserve special care: published cases describe vancomycin concentrations sampled through a port reading much higher than peripheral samples. Draw levels from a different lumen or a peripheral vein when policy allows, and record the source.

Apply the steps to a study scenario

In a hypothetical item, a patient with a triple-lumen catheter receiving dextrose-containing fluids has a glucose result far above any previous value, without symptoms. Options include giving insulin by sliding scale, repeating the value immediately by fingerstick or a correctly collected sample, or increasing the infusion rate. Verifying the result first is correct because the pattern suggests contamination.

Insulin would treat a number that may not reflect the patient and could cause hypoglycaemia. Report the discrepancy to the prescriber and laboratory, document how the sample was obtained and review technique with the team. Exam questions reward recognising that an unexpected result needs a valid specimen before treatment, especially when the sampling route offers an explanation.

Sources and further reading

Journal of Laboratory Physicians (PMC): 3 mL vs 5 mL discard for blood sampling from central venous access devices. Rationale for the discard method and evidence that a smaller discard performed comparably in ICU patients.

ADLM Scientific Short: What percentage of your blood specimens are contaminated with IV fluid?. How IV fluid contamination occurs, its effect on glucose and electrolytes, and prevention measures.

British Journal of Clinical Pharmacology (PMC): Falsely elevated vancomycin concentrations sampled from portacaths. Cases of drug levels from central devices reading higher than peripheral samples.

Joint EFLM-COLABIOCLI Recommendation for venous blood sampling. Catheter sampling discouraged because of haemolysis and admixture with intravenous or flush fluid.