Blood samples for ammonia analysis do not require transport to the laboratory on ice: a study of ammonia stability and cause of in vitro ammonia increase in samples from patients with hyperammonaemia.

Abstract

Objectives: Prompt recognition of hyperammonaemia can avoid severe consequences of delayed treatment. Strict sample transport requirements present barriers to requesting and, if not achieved, rejection by the laboratory. Evidence is sparse on in vitro ammonia stability from studies using modern techniques or based in clinical settings. Stability in hyperammonaemic samples is unknown. This study aimed to examine ammonia stability and its source in samples from hyperammonaemic patients and to determine a clinically significant change to establish acceptable sample requirements for ammonia analysis.

Methods: Blood samples were taken from 19 hyperammonaemic patients and placed either on ice or kept at room temperature. Plasma ammonia was measured every 10 min for 2 h. Haemolysis index (HI), full blood count, liver enzymes and amino acids were analysed. Expert physicians were surveyed on a clinically significant ammonia change. Stability was assessed using the reference change value (RCV).

Results: Ammonia increased with time [peak value 14.9 % (8.4-17.1), median (95 % confidence interval)], and was predominately of cellular origin. Ice did not improve stability and increased HI. Survey results found a significantly increased ammonia between 39 % (30-48) at 50 μmol/L and 21 % (15-28) at 1,000 μmol/L. Ammonia RCV was 40.8 %.

Conclusions: Chilling samples did not improve blood ammonia stability. The increase in blood ammonia from patients with hyperammonaemia over 2 h was lower than that considered clinically significant and the calculated RCV. Transport of samples for ammonia analysis does not require ice and laboratories should accept samples if received within 2 h of venepuncture.