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Jan. 1, 2011

The impact of permissive hypercapnia on laboratories that perform blood-gas analysis likely goes beyond just knowing what values to expect
from a given patient population. All laboratories are required to define critical values — values which are generally handled differently
(i.e., repeated before reporting) and must be reported to the responsible caregiver within a defined period of time. Blood-gas laboratories within institutions that practice permissive hypercapnia may see many highly unusual pCO2 values.  If the laboratory will define a critical pCO2 value, it is likely that many patients on permissive hypercapnia will have critical results. Similar to the case of the nephrology service not wanting critical creatinine values called to them at all hours of the day and night, critical-care physicians and others may lobby the laboratory to drop the critical value for pCO2 or ask for an exemption from critical call-backs for this parameter. The laboratory — in consultation with the medical staff — will have to decide what action to take (e.g., drop pCO2 from critical list, make an exemption by patient location, make an exemption by ordering physician); depending upon the reliability of follow-up actions for unexpected high pCO2 values. To make the right decision, it will be important to understand the practice at your institution (i.e., how many very high pCO2 values are being generated from patients on permissive hypercapnia vs. unexpected high pCO2 results?). In addition, some labs define “clinical action values” that do not generate a critical call but require some action (e.g., repeat analysis, confirmation by alternate method) before results are released. Again, institutions with many patients on permissive hypercapnia may want to avoid using pCO2 as a clinical action value as they will frequently repeat tests or perform confirmation for results that are expected for the patient population.

Brad S. Karon, MD, PhD
Hospital Clinical Laboratories
Mayo Clinic
Rochester, MN