4.30: Ammonia Analysis
- Page ID
- 122449
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The laboratory receives a specimen from the Neonatal Intensive Care Unit (NICU) for an ammonia measurement. The analysis is performed on a centrifugal analyzer using an enzymatic method (glutamate dehydrogenase, GLDH). The result is reported as 1170 mmol/L (upper limit of the reference range is 72 mmol/L).
A second sample from the same infant is received later that evening on third shift. Because ammonia levels cannot be performed on third shift, the sample is sent to a nearby hospital that does have stat. ammonia analysis capabilities. The other hospital also employs an enzymatic method for measuring ammonia levels, but uses a different analyzer. The result for this sample is 48 mmol/L.
The NICU physician calls the laboratory wanting an explanation for the difference between results.
QUESTIONS
- What can the laboratory do to confirm these results?
- The laboratory supervisor asks a technologist to retrieve both specimens and re-analyze them for ammonia. After verifying the infant’s identification on both specimens, the technologist measures the ammonia levels, reporting results of 1245 and 67 mmol/L for the first and second results, respectively.
The laboratory supervisor talks with the NICU physician and determines that the ammonia levels were requested because of the infant’s failure to thrive; the physician wants to rule out an inborn error of metabolism as the cause of the infant’s problems. The physician does not know of any clinical state that would cause such a rapid change in ammonia levels. A third sample is obtained from the infant, and the ammonia level is found to be normal.
The laboratory’s clinical chemist asks the supervisor to find out what kind of tubes were available in the NICU for obtaining the samples for ammonia analysis. The supervisor reports back that heparinized capillary tubes were available for the initial analysis but that they contained neither sodium nor potassium heparin. The second and third specimens were drawn into small vacutainer tubes containing sodium heparin. What might be the cause of the apparently falsely elevated ammonia level in the first specimen?
Questions to Consider
- What is the principle of analysis of the enzymatic method for ammonia that employs the same GLDH reaction employed in the enzymatic measurement of urea?
- What other methods are available for ammonia measurements?
- For what clinical problems would an ammonia measurement be requested?
- What are the specimen requirements for an ammonia measurement?
- What kind of phlebotomy tube would be used for an Infant?
- Answer
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- The laboratory should verify these results by repeating the ammonia analyses on both specimens. If possible, the first specimen should be analyzed by the other hospital’s GLDH procedure. It would probably be useful to request a third sample. The repeat analyses might be significantly higher than the initial results because the samples are unstable with time, generating ammonia-like substances upon sitting.
- The most likely explanation for the elevated ammonia levels in the first sample was the use of ammonium heparin as the anticoagulant in the capillary tube.
Answers to Questions to Consider
- The GLDH reaction converts alpha-ketoglutarate to glutamic acid in the presence of, NADH, and NH4+ ((Link to Methods CD Ammonia). As the reaction proceeds, NADH is oxidized to NAD+, and the decrease in absorbance at 340 nm is directly related to the ammonia concentration.
- Another method available for the measurement of ammonia is a potentiometric one, using an ion-selective electrode (p. 277).
- Ammonia levels can be highly elevated in severe liver disease, such as seen in Reye’s syndrome, liver cirrhosis, and hepatic failure (Chapter 27). Hyperalimentation can also cause a hyperammonemia (Chapter 27). A rare inborn error of metabolism that affects the urea cycle can cause a marked elevation in blood ammonia levels. These infants are usually labeled “unable to thrive” as a result of the genetic abnormality (Chapter 47).
- Ammonia is usually measured in plasma or whole blood, using heparin as the preferred anticoagulant. The specimen must be protected from air and be refrigerated. If analysis can not be performed with a few hours, the sample should be frozen.
- In order to minimize the volume of blood taken from the infant, a capillary tube would probably be used for phlebotomy (p. 70).