5.2 Analytical Interference Factors

5.2 Analytical Interference Factors

Analytical interference occurs when something in the specimen—or something introduced during collection—alters the measured value of an analyte without reflecting the patient's true physiologic state. For the AMCA PTC candidate, the key insight is that most analytical errors begin as pre-analytical technique problems. Understanding how hemolysis, lipemia, icterus, hemoconcentration, IV contamination, additive carryover, and drugs distort results helps you prevent rejection, redraws, and dangerous clinical decisions.

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Hemolysis, Lipemia, and Icterus

These three visual conditions are the classic specimen interferences flagged by laboratory staff and automated analyzers.

Hemolysis

Hemolysis is rupture of red blood cells with release of hemoglobin and intracellular contents into plasma or serum. Visually, the serum or plasma appears pink to cherry red. Causes tied directly to phlebotomy include:

• Using a needle that is too small for the vacuum (excessive shear force)
• Forcing blood through a syringe needle into tubes
• Vigorous shaking instead of gentle inversion
• Drawing through a hematoma or probing repeatedly
• Leaving the tourniquet on too long and then collecting under high venous pressure
• Incomplete drying of alcohol before puncture

Analytes commonly falsely elevated by hemolysis include potassium (K+), lactate dehydrogenase (LDH), aspartate aminotransferase (AST), magnesium, and phosphate, because these substances are concentrated inside RBCs. Hemoglobin itself can interfere with colorimetric and spectrophotometric assays, producing falsely high or low results depending on the method.

Lipemia

Lipemia is excess lipid (usually triglycerides) that makes plasma or serum milky or turbid. It is often seen after a fatty meal, in uncontrolled diabetes, or with parenteral nutrition. Lipemia scatters light and interferes with many photometric assays. Collection tip: when a fasting specimen is ordered, verify fasting status before the draw so the laboratory is not forced to report interference or request a redraw.

Icterus

Icterus is yellow discoloration from elevated bilirubin. Severe icterus can interfere with assays that use yellow or orange wavelengths. Unlike hemolysis, icterus is usually patient-related rather than technique-related, but the phlebotomist still protects quality by collecting adequate volume, protecting light-sensitive bilirubin specimens when required, and avoiding hemolysis that compounds the problem.

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Hemoconcentration From Prolonged Tourniquet Use

A tourniquet slows venous return and pools blood distal to the constriction. After about one minute, fluid and small molecules filter into the tissues while larger molecules and cells remain in the vessel. This hemoconcentration falsely elevates:

• Potassium
• Total protein and albumin
• Cholesterol and other lipids
• Hematocrit and related cellular indices (to a lesser degree)

Best practice: apply the tourniquet, locate the vein promptly, cleanse and allow drying, then insert. Release the tourniquet as soon as blood flow is established, and never exceed one minute of continuous application. If vein location takes longer, release for two minutes and reapply before puncturing.

IV Fluid Contamination

Drawing from an arm with an active intravenous (IV) infusion risks diluting the specimen with IV fluid or contaminating it with the infused drug or electrolyte. Classic errors include:

• Drawing above a running IV → specimen diluted with saline or D5W; falsely low electrolytes, hematocrit, and many chemistries; falsely high glucose if dextrose is infusing
• Drawing immediately after a medication push without adequate flush and discard

Safe approach (facility policy governs details):

1. Prefer the opposite arm.
2. If the IV arm must be used, stop the infusion (with nurse authorization when required), wait the policy-specified interval, draw below the IV site, and discard an initial volume when drawing from a line.
3. Never draw above a running IV for chemistry or hematology panels.

Document the draw site relative to any IV so the laboratory can interpret unexpected values.

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Wrong Additive Carryover

Evacuated tubes contain precise additives. If order of draw is violated, residual anticoagulant can contaminate the next tube:

• EDTA carryover into a serum chemistry tube → falsely low calcium (EDTA chelates calcium), falsely high potassium
• EDTA or heparin into a citrate coagulation tube → invalid PT/aPTT ratios
• Clot activator or serum tube contents into citrate → premature clotting activation

Prevention is procedural: follow CLSI order of draw, fill tubes to the fill line, invert gently the correct number of times, and never pour blood from one tube into another.

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Drug Interferences Overview

Medications and contrast agents can alter measured analytes or assay reactions. Examples phlebotomists should appreciate (not memorize as a pharmacy list):

• Antibiotics, anticonvulsants, and other drugs measured as therapeutic drug monitoring (TDM) levels—timing of the draw is critical
• Biotin (high-dose supplements) can interfere with some immunoassay platforms
• Radiographic contrast may affect certain chemistry methods shortly after administration
• Heparin or warfarin therapy does not spoil every test, but it changes coagulation results and prolongs bleeding time after puncture

When requisitions list TDM, peak/trough, or draw before next dose, coordinate with nursing. If a patient reports a new high-dose supplement or recent contrast study and the lab questions a result, document what you know.

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How Phlebotomy Technique Prevents Analytical Error

Use this prevention checklist as your mental model for Task 4.03:

1. Identify and verify fasting/timed status before collecting.
2. Limit tourniquet time to ≤1 minute; release early.
3. Allow antiseptic to air-dry completely to reduce hemolysis and contamination.
4. Choose correct needle gauge and avoid forcing syringe blood into tubes.
5. Follow order of draw and fill tubes to the proper level.
6. Invert gently—never shake.
7. Avoid IV-contaminated sites; prefer opposite arm.
8. Protect light- and temperature-sensitive specimens per lab instructions.
9. Transport promptly and keep tubes upright when required.
10. Label at the bedside so the right results attach to the right patient—misidentification is the ultimate analytical disaster.

Analytical interference is not only a laboratory problem. It is often a collection problem that skilled phlebotomy can stop at the chair.