What is the most suitable specimen for ammonia measurement?

Aims: We studied the suitability of collection tubes on the stability of ammonia in vitro.Methods: Blood was collected from five volunteers into EDTA, heparin and serum tubes on ice. Blood was allowed to clot in serum tubes (clot activator) for 10 minutes. For each specimen type, 11 aliquots were made. Ammonia was measured in one aliquot at 15 minutes after collection. Five aliquots were stored at 4°C and five frozen at –20°C: ammonia was measured at 2, 3, 4, 8 and 24 hours after collection. Measurement was performed on Beckman AU 5800 analyser.Results: Mean baseline ammonia concentration was 26 μmol/L in EDTA plasma, 24 μmol/L in heparin plasma and 34 μmol/L in serum. For specimens kept at 4°C for 2 hours, the mean increase was 4 μmol/L in EDTA, 5 μmol/L in heparin, 7 μmol/L in serum. For specimens kept at 4°C for 24 hours, the mean concentration was 36 μmol/L in EDTA and heparin, 61 μmol/L in serum. Freezing did not improve specimen stability at 24 hours except in serum.Conclusion: EDTA and heparin tubes are suitable for ammonia measurement. Serum is unsuitable even when collected on ice and frozen as the increase in ammonia starts with blood clotting process. Aims: We studied the suitability of collection tubes on the stability of ammonia in vitro. Methods: Blood was collected from five volunteers into EDTA, heparin and serum tubes on ice. Blood was allowed to clot in serum tubes (clot activator) for 10 minutes. For each specimen type, 11 aliquots were made. Ammonia was measured in one aliquot at 15 minutes after collection. Five aliquots were stored at 4°C and five frozen at –20°C: ammonia was measured at 2, 3, 4, 8 and 24 hours after collection. Measurement was performed on Beckman AU 5800 analyser. Results: Mean baseline ammonia concentration was 26 μmol/L in EDTA plasma, 24 μmol/L in heparin plasma and 34 μmol/L in serum. For specimens kept at 4°C for 2 hours, the mean increase was 4 μmol/L in EDTA, 5 μmol/L in heparin, 7 μmol/L in serum. For specimens kept at 4°C for 24 hours, the mean concentration was 36 μmol/L in EDTA and heparin, 61 μmol/L in serum. Freezing did not improve specimen stability at 24 hours except in serum. Conclusion: EDTA and heparin tubes are suitable for ammonia measurement. Serum is unsuitable even when collected on ice and frozen as the increase in ammonia starts with blood clotting process.