 |
| Obtaining Accurate Blood Ammonia Measurements by Julian Williams, M.D., Division Head of Medical Genetics, Children's Hospital of Los Angeles Accurate measurement of blood ammonia is essential to the diagnosis and management of hereditary enzyme deficiencies in the urea cycle. Unfortunately, accurate measurements are difficult to obtain secondary to improper blood drawing and delays in performing the ammonia analysis. The key to obtaining good values is a realization that tissue degradation results in ammonia release. Thus, maceration of tissue/blood or letting it sit at room temperature results in elevated ammonia values even in a patient without a urea cycle enzyme deficiency. TECHNIQUE The blood sample should be drawn with as large a needle as possible. A 25 or 27 gauge needle has such a small internal diameter that the blood cells can be broken by drawing blood through it or by squirting blood from the syringe through the needle into a test tube. Ideally, unless the needle is large, after drawing the sample the test tube top and the needle should be removed and the blood gently added to the green top tube (heparanized). The minimum number of needle sticks, ideally, one, should also be used to obtain the blood sample. Multiple needle sticks destroy small amounts of tissue and microscopic pieces lodge in the needle or are present in the blood sample. This can raise a normal blood ammonia (10-50 umoles/liter in our hospital) to 200-300 umoles/liter. Heel or finger sticks can never be relied on to obtain accurate values. Once obtained, the blood sample should be placed on ice and run STAT - that is, get the results in one hour or less. Letting the blood sample sit at room temperature will increase blood ammonia 10 - 20 umoles/liter for every hour that it sits. SUMMARY Following these simple guidelines will ensure accurate measurements so that appropriate therapy can be given. Although we are very aggressive in treating urea cycle disorders, aggressive therapy always has some risks and should be used appropriately. Also, assuming all blood ammonia levels are falsely high could result in delay in appropriate treatment when the ammonia is actually high. This could be a tragedy. (Reprinted from National Urea Cycle Disorders Foundation Newsletter, Winter 1996) |
|