What is the recommended method for ammonia sampling in a clinical or occupational health context?

Ammonia Sampling: Recommended Method

For clinical ammonia sampling, collect venous blood in an EDTA tube from a fasting patient, place immediately on ice, separate plasma from cells within 15 minutes, and analyze within 60-90 minutes to ensure accurate results. 1, 2

Critical Pre-Analytical Requirements

The accuracy of ammonia measurement depends entirely on proper sample handling, as ammonia is extremely labile and unstable. 1

Collection Technique

• Draw from a free-flowing venous sample without venous stasis—avoid tourniquet use and fist clenching, as these falsely elevate results 1, 2
• Collect from fasting patients when possible to minimize postprandial elevation (ammonia increases 12-18% within 1-2 hours after protein meals) 1, 3
• Clean skin thoroughly before venipuncture 1
• Use EDTA-anticoagulated tubes (lithium heparin is acceptable as second choice)—EDTA has been demonstrated superior to heparin or oxalate anticoagulants 1, 2
• Fill tube completely, secure lid tightly, and homogenize by inversion 1, 2

Immediate Post-Collection Handling

• Place sample on ice immediately (or synthetic ice) after collection 1, 2
• Transport to laboratory at +4°C within 15 minutes of collection 2
• Separate plasma from cells within 15 minutes to prevent continued ammonia metabolism by red blood cells 2
• Analyze immediately after plasma separation—maximum acceptable delay is 60-90 minutes from collection to analysis 1, 2

Common Pitfalls and Interference

Several factors produce falsely elevated ammonia levels that must be avoided:

• Hemolysis releases ammonia from red blood cells and invalidates results 1, 2
• Delayed processing beyond 2 hours causes progressive ammonia elevation from ongoing cellular metabolism 1
• High lipemia and jaundice interfere with photometric measurement 1
• Smoking, exercise, and certain medications elevate baseline ammonia 1
• Multiple freeze-thaw cycles profoundly affect frozen samples 1

If delayed measurement is unavoidable, centrifuge at +4°C and freeze the supernatant at −70/−80°C, though fresh analysis is strongly preferred. 1, 2

Clinical Context for Interpretation

While proper sampling technique is essential, understanding the clinical utility of ammonia measurement is equally important:

• A normal ammonia level (<35 μmol/L in adults) casts doubt on hepatic encephalopathy diagnosis and should prompt investigation for alternative causes 1, 2, 4
• Elevated ammonia alone does not confirm hepatic encephalopathy, as cirrhotic patients may have elevated levels without encephalopathy 1, 4
• Ammonia >100 μmol/L with minimal liver dysfunction should raise suspicion for inherited metabolic disorders, particularly with family history of liver or neurological disease 1, 4
• Serial measurements can be useful for monitoring treatment response in ammonia-lowering therapy 1, 4

Measurement Method

The standard enzymatic kinetic assay measures ammonia through its reaction with α-ketoglutarate and NADPH to form glutamate and NADP+, with ammonia concentration determined by photometric measurement of NADPH oxidation. 1 However, this method's extreme sensitivity to pre-analytical variables makes proper sample handling more critical than the specific analytical technique used.