From the Guidelines
Blood ammonia levels are measured through an enzymatic kinetic assay, where ammonia reacts with α-ketoglutarate and NADPH to form glutamate and NADP+, with the amount of ammonia related to the amount of NADPH oxidized, which is measured photometrically 1. This method is the most commonly used and involves measuring the amount of NADPH oxidized, which is directly related to the amount of ammonia present in the blood.
- The measurement of ammonia is extremely sensitive and requires careful control of pre-analytical events such as delayed sample processing, temperature, type of test matrices, hemolysis, and storage time in the freezer.
- To ensure accurate results, it is recommended to place the sample on ice and immediately remove the plasma from cells to reduce the reaction rate of ammonia metabolism.
- The delay between sampling and analysis should be kept within 2 hours, and heparin, EDTA, and oxalate anticoagulated plasma have been tested, with EDTA-anticoagulated matrix found to be superior 1.
- Frozen samples can affect ammonia levels, and multiple freezing and thawing cycles can have a profound effect, highlighting the importance of proper specimen handling.
- Measuring brain tissue ammonia is further challenging due to multiple pre-analytical steps, but alternative methods such as cerebrospinal fluid and microdialysate measurements of ammonia have been performed, which are believed to better reflect brain ammonia concentrations 1.
From the Research
Measurement of Ammonia in Blood
- Ammonia levels in blood can be measured using various methods, including a specific ion-selective electrode system or an automated enzyme method, such as a glutamate dehydrogenase (GLDH)-based assay 2, 3.
- The choice of anticoagulant is important, as heparin-anticoagulated plasma has been shown to be less precise than EDTA- and oxalate-anticoagulated plasma 3.
- The stability of ammonia in blood samples is also crucial, with studies showing that plasma kept at -70°C for up to three weeks shows no change in measured ammonia relative to the baseline determination 3.
- The collection, handling, storage, and analysis of blood samples are critical to obtaining accurate results, and recommendations have been made on the collection and processing of blood samples to minimize the release of ammonia from the collected sample before analysis 4.
Clinical Use of Ammonia Measurement
- Measurement of plasma ammonia is important in the diagnosis and management of inherited metabolic disorders, particularly in neonates and pediatric patients 2.
- Elevated ammonia levels in blood can indicate an abnormality in nitrogen homeostasis, and can be associated with a range of clinical conditions, including hyperammonemic disorders and liver disease 5, 6.
- The threshold for accepting requests for plasma ammonia measurement should be lower than at present to prevent misdiagnoses, particularly in patients with unexplained mental status changes or coma 2.