| January 1998
Welcome to the 1998 issues
of ANTECH NEWS! Measuring CO2 Accurate, clinically relevant results depend upon specimen integrity. Antech’s multifaceted approach to improve the standards for clinical testing began last year: we offered two newsletters about specimen integrity, updated several assay techniques, introduced new tests, and provided specimen materials to facilitate transport to the lab (aprotinin tubes for endogenous ACTH samples, urine and viral culture transport vials, cassette holders for smaller tissues). Now we inform you of the shortcomings in the routine analysis of total carbon dioxide (CO2), caused by problems related to specimen collection and transport. Over the past year, we have had numerous con plaints from veterinarians about the lack of correlation between total CO2 levels and clinical findings. After considerable investigative efforts, we deter- mined that these sporadic problems were directly related to the following conditions:
These problems with CO2 analysis have been documented in the recent literature. Summary Although Antech uses quality reagents and appropriate standards, controls, and equipment, over 50% of the specimen tubes we receive are less than optimally filled. This provides an accurately measured value but one which is often clinically erroneous. Faced with this problem, we are removing CO2 from our routine panels effective February 1. 1998. If there is a specific need for a CO2 assay on an animal, add it to the chemistry screen by writing CO2 in the requisition add-on column. The assay will be performed at no charge. If you wish to continue to receive CO2 levels on all chemistry samples, please notify your customer service representative or laboratory manager by February 1, 1998. Please assure that specimens for CO2 analysis are drawn into completely filled gel separator tubes, centrifuged and cooled during transport. The CO2 analysis are drawn into completely filled gel separator tubes, centrifuged and cooled during transport. The CO2 value will be accurately measured, but it may not correlate with your clinical findings. Bibliography for CO2 Analysis
The author drew 10, 3 or 1 mL of blood into 10 mL tubes. Mean total CO2 values were 2.0 and 3.7 mEq/L greater for the full 10 mL tube than for the 3 mL and 1 mL filled tubes.
The author also drew 10, 3 or 1 mL of blood into 10 mL tubes, Total CO2 levels were 21.7 (10 mL), 19.4 (3 mL) and 16.3 (1 mL) mEq/L, a finding con- firmed later by the James et al study.
Mean total CO2 levels using an enzymatic method (18.7 mEq/L) were substantially lower than a direct measurement with an electrode (22.2 mEq/L).
Review of Acid/Base Physiology & CO2 The bicarbonate level of blood plays an essential role in acid-base balance and is reflected by the serum total CO2 level. Acid-base balance is maintained through renal and pulmonary mechanisms. Oxidative metabolism produces large amounts of CO2 each day. On combining with water in the blood stream, CO2 Forms carbonic acid. This reaction is facilitated by the enzyme carbonic anhydrase from red blood cells. The excess hydrogen formed is buffered by intracellular constituents such as hemoglobin. The bicarbonate leaves the erythrocyte and enters the extra- cellular fluid in exchange for extracellular chloride. The net effect of these reactions is to carry CO2 in the bloodstream as bicarbonate with little change in extrace11ular pH. These processes are reversed in pulmonary alveoli, as CO2 is excreted by ventilation. ACID-BASE IMBALANCES AND COMPENSATORY MECHANISMS
The body’s buffering capacity includes the extra- and intracellular buffers and bone. Extracellular buffers are the bicarbonate (HCO3 -/H2CO3) and phosphate (HPO4 2-/H2PO4 ) buffer pairs, and the plasma proteins, organic and inorganic phosphates, and hemoglobin. Bone carbonate provides a large and generally overlooked buffer store which is estimated to contribute up to 40% of the buffering capacity of an acute acid load. A change in hydrogen ion concentration affects all buffer pairs in the body and hence measurement of one buffer pair reflects changes in the others . Serum/plasma pH is determined by the ratio between bicarbonate and carbonic acid concentration. Respiratory acidosis or alkalosis depends upon CO2 levels, while metabolic acidosis or alkalosis is determined by HCO3–. Primary respiratory imbalances in acid-base metabolism are counter- balanced by compensatory changes mediated through the kidneys which alter the excretion or retention of hydrogen ions or bicarbonate. The most commonly used clinical assay measures total CO2, the combination of CO2 and HCO3 . Elevated CO2 can arise from either respiratory acidosis or metabolic alkalosis. Therefore, accurate assessment of acid- base balance requires information about pH and pCO2 as well. Unfortunately, the accuracy of these assays depends upon their rapid assessment, so that transport and time delays often render them invalid. Urine colony counts or quantification of urine cultures determines the number of bacterial colonies per mL of urine. This is helpful in assessing whether the bacteria isolated and identified on culture are possible contaminants, normal flora, or significant pathogens. This test is performed by taking a precisely measured amount of urine from a sterile container and streaking it on isolation media. After routine incubation, the number of colonies of bacteria for one or more isolates is counted and calculated per mL of urine. In order to correctly interpret the results of the test, certain collection procedures must be followed.
Results are reported as the number of colonies (or the range) per mL of urine along with the identification of the organism and antimicrobial sensitivity. Less than 1,000 bacterial per mL of urine is considered insignificant, if the pre- ceding guidelines are followed. For urines collected by mid- stream catch or catheterization, less than 10,000 per mL is insignificant. Also, mixed bacterial isolations (2 or more different bacteria) are indicative of contamination and do not accurately reflect the urinary tract infection. This is true for urine colony count as well as for direct plating techniques. The highest reported value for bacterial counts is greater than 100,000/mL. Urine colony count may be ordered by requesting URJNE COLONY COUNT – Test Code 0 3796. This includes colony count, identification and sensitivity. Turnaround time is 48 hrs.
LAB TIPS For endogenous ACTH determination, be sure to separate the plasma from the blood cells when using the special aprotinin tubes provided by Antech. Aprotinin use avoids the necessity to freeze the sample for accurate results; it does not replace the need for centrifugation to separate the plasma. Patients admitted in acute bleeding crisis may have coagulation screening test values (PT, PTT) within upper normal ranges or only slightly elevated because of stress increase in the fibrinogen and other acute-phase procoagulants. Thus, diagnosis of coagulopathy may be misleading or overlooked. Patient history (age, sex, neuter status, family, breed, current or prior events, toxin or drug exposures) and nature of bleeding (hematomas, mucosal surfaces, petechiae) are of primary concern. Once bleeding is con- trolled and resolved, a repeat coagulation profile is advised. |
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