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Low pH control.

Q I am having a difficult time answering several questions on the CAP checklist for blood gases. At our hospital, it is not uncommon to have pHs in the 6.6 or 6.7 range. The blood-gas instrument has a measuring range of 6.3 to 8.0. I cannot find any kind of control material that can test that range. Most commercially prepared material does not go lower than 6.8. I can report <6.8, but doctors in the special-care nursery would not like it. In reporting less than or greater than, how does that affect calculated results based on those tests? Also, CAP checklist question BGL.29770 asks: "Do the control materials cover the analytical measurement range (AMR) of the analyzer?" No blood-gas control can cover the AMR.

A The first issue concerns the reporting of blood-gas results that are in compliance with CAP question BGL.29770. This requirement asks: "Do the control materials cover the analytical measurement range of the analyzer?" Many laboratorians interpret this as a requirement for the use of quality-control material that spans the lower and upper analytical limits of measured blood-gas parameters that might be reported from patient specimens. For example, an instrument with an analytic measurement range of 15 mm to 600 mm Hg for p[O.sub.2] would require the use of quality-control material that spans these limits if patient

p[O.sub.2] values, measured at these extreme limits, were to be reported.

Fortunately, this literal interpretation of BGL.29770 is not required for compliance with this CAP question. Discussion with a technical specialist at CAP (V. Emmons, MT[ASCP], written communication, October 2002) concerning the interpretation of BGL.29770 indicates a more reasonable approach for complying with this requirement. According to CAP, control material containing a combination of high, low, and normal values should be assayed each day of testing. Minimum acceptable laboratory practice for analysis of quality-control material, in accordance with CAP question BGL.29760, requires that two levels of quality-control material be run for each eight hours of testing. It is not required that control values extend to the end points of the analytical-measurement range. Controls must span the analytical-measurement range in a reasonable way, with control values near important clinical-decision levels.

Recommended critical-decision levels for blood-gas parameters measured in arterial or capillary blood include lower and upper limits, respectively, of 7.20 and 7.60 for pH and 20 mm and 70 mm Hg for pC[O.sub.2]. For p[O.sub.2], a lower limit of 45 mm Hg for p[O.sub.2] measured in arterial blood and a critical lower limit of 20 mm Hg for p[O.sub.2] measured in capillary blood has been recommended. (1) Thus, compliance with BGL.29770 is readily achieved by the appropriate use of quality-control material in a manner similar to that employed in the evaluation of other laboratory parameters.

The second issue concerns the effect that reporting a value as less than some pre-defined cutoff limit has on calculated blood-gas parameters. Blood-gas instruments calculate or estimate parameters, including oxygen saturation, oxygen binding capacity, and p50, by incorporating measured parameters such as pH, pC[O.sub.2], and p[O.sub.2], into various calculations. (2) Reporting pH, pC[O.sub.2], or p[O.sub.2] values as less than or greater than some threshold value should not affect the calculated parameter, since the instrument will use the actual measured value in the calculation. It should be stressed, however, that many of the calculated or estimated parameters reported as part of a blood-gas analysis assume that the values for the dissociation constant (pK) for carbonic acid, solubility coefficient (~) for C[O.sub.2], [O.sub.2] affinity of hemoglobin, and 2- and 3-DPG levels are those that are observed in normal, healthy patients. Unfortunately, these values can be altered significantly in patients with certain pathologic conditions. Ill patients and some children with abnormalities in measured pH, pC[O.sub.2], or p[O.sub.2], or who have significant abnormalities in these constant values, may have calculated blood-gas parameters that differ significantly from the actual value. (3) Thus, extreme caution must be exercised in using parameters calculated from blood-gas analyzers in extremely ill patients and children.

References

1. Kost GJ. Critical limits for urgent clinical notification at US medical centers. JAMA. 1990;263:704-707.

2. Natelson S, Nobel D. Effect of the variation of pK of the Henderson-Hasselbach equation on values obtained for total C[O.sub.2], calculated from pC[O.sub.2] and pH values. Clin Chem. 1977;23:767-769.

3. Rosan RC, Enlander D, Ellis J. Unpredictable error in calculated bicarbonate during pediatric intensive care: The exclusion of fixed pK. Clin Chem. 1983;29:69-73.

--Steven C. Kazmierczak, PhD, DABCC

Professor, Department of Pathology

Oregon Health and Science University

Portland, OR

Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.

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Title Annotation:Answering your questions
Author:Kazmierczak, Steven C.
Publication:Medical Laboratory Observer
Date:Nov 1, 2004
Words:841
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