Technical pitfalls in measurement of venous plasma NH3 concentration.

Abstract

Measurement of venous plasma NH3 in normal subjects by the ion-exchange method of Forman [Clin. Chem. 10, 497 (1964)] in a hospital clinical laboratory gave a mean value of 640 mug/liter (range, 300-1320 mug/liter; intraassay, intra-individual, and inter-individual coefficients of variation, 8, 47, and 47%, respectively). The following conditions adversely affect the reproducibility of the test: pollution of laboratory atmosphere and glassware by NH3-containing detergents; smoking by patient or analyst; delay, turbulence, or use of heparin lock in venipuncture; delay or warming of plasma above degrees C before mixing it with resin; and delay in colorimetric analysis of resin eluate. When these sources of error were eliminated, the mean value for normals was reduced to 330 mug/liter, the range was narrowed to 220-470 mug/liter, and the above-mentioned CV's were 5, 16, and 17%, respectively. With the precautions cited, furthermore, the intra-assay and intra-individual CV's for fasting NH3 concentration in cirrhotic patients were similarly reduced. An NH3 tolerance test was done by administering a standard dose of NH4Cl to patients and measuring venous plasma NH3 at 0, 15, 30, 60, and 90 min; the NH3 tolerance was quantified from the area under the curve relating concentration to time (mug - min/liter). As measured in the clinical laboratory, NH3 tolerance of cirrhotic patients showed intra-assay and intra-individual CV's of 50 to 90%. When the tolerance tests were repeated in the same subjects with the laboratory precautions listed above, these CV's were reduced to 8-15%.

Diagnostic Value of Fasting Plasma Ammonia and Bile Acid Concentrations in the Identification of Portosystemic Shunting in Dogs

“…It is thus impossible to send samples to a specialized laboratory by mail. [42][43][44][45][46] Although several authors mention this problem as the main reason for avoiding the use of AMM tests, this disadvantage has been largely resolved by the introduction of the Blood Ammonia Checker II. e This analyzer proved to be a simple, reliable, and relatively inexpensive alternative to the standard automated enzymatic assay.…”

Blood Ammonia Stability Revisited

“…A nontraumatic, non-probing venipuncture was required for an acceptable specimen for this study. 3 In addition to the blood collected for the physician-ordered tests, two 3mL evacuated lithium heparin tubes (BD Vacutainer®) were collected from each individual and labeled with pre-printed study number labels. Immediately after labeling, one of the extra tubes was placed in the side pocket of a 5"x9" biohazard bag filled with approximately one cup of ice, and the second extra tube was maintained at room temperature (23°C) in a 5"x9" biohazard bag.…”

“…1 Numerous studies have shown that plasma ammonia levels increase in vitro over time and may be falsely elevated if the specimen is not chilled "on-ice" prior to processing and analysis. [1][2][3][4][5][6][7] The sources of the increase in plasma ammonia concentration in vitro reportedly include deamination reactions in erythrocytes and platelets, and the deamidation of plasma glutamine by γglutamyltransferase (γ-GT). 4,6,[8][9][10] While these studies demonstrated the clinical importance of plasma ammonia specimen integrity, many of these studies were performed with blood collection techniques and analytical methods that are no longer in use.…”

“…1,6 Other studies utilized microdiffusion or ion-exchange methods to determine ammonia concentration, which are methods not used today. 1,3 Furthermore, previous studies focused on storage of specimens for hours to days prior to ammonia analysis and various temperature conditions for storage. [1][2][3][4][5][6] Based upon the aforementioned studies, the established procedure at Parham Doctors' Hospital (Hospital Corporation of America, Richmond, VA) requires that any blood specimen collected for plasma ammonia measurement not received "on ice" be rejected, regardless of how soon after collection the specimen is received in the laboratory.…”

Blood Ammonia Determination in Cirrhosis: Still Confusing After All These Years?

“…In patients with liver disease, one cigarette smoked 1 hour before venipuncture was reported to increase fasting blood ammonia levels by 10 umol/L. 20 It is obvious that ammonia determinations have to be performed in a laboratory environment free of open urine samples or open bottles containing high ammonia standards. Ong et al 13 used an enzymatic method whereas Nicolao et al 14 used the Blood Ammonia-Checker II (BAC-II; Menarini, Florence, Italy) for ammonia measurement.…”