A 56-year-old diabetic male is brought to the emergency department due to nausea and vomiting. While you are examining the patient, a nurse asks if determination of the patient's acid-base status will help ascertain the etiology and subsequent management of the patient's primary problem. Which of the following pairs of laboratory values will help get the best picture of the patient's acid-base status? A. pH and PaCO2 (88%) B. pH and PaO2 (1%) C. PaO2 and PaCO2 (0%) D. Urinary pH and PaCO2 (1%) E. Urinary pH and HCO3- (7%) Incorrect Correct answer A 88% Answered correctly 44 secs Time Spent Explanation The major extracellular buffer in human blood is the carbon dioxide - bicarbonate buffer pair, which has a pK of 6.1. Acid-base disturbances in patients can be diagnosed by assessing the pH and the changes in either the serum bicarbonate concentration or the serum partial pressure of carbon dioxide. Classically, the acid-base status of a buffer can be determined using the Henderson-Hasselbalch equation with its three variables, pH, [acid] and [conjugate base], as follows: pH = pK + log ([conjugate base] / [acid]) In the case of the carbon dioxide - bicarbonate buffer pair, where CO2 is the acid and HCO3- is the conjugate base, the equation is as follows: pH = 6.1 + log ([HCO3-]/(0.03 x PaCO2)) Using the Henderson-Hasselbalch equation, any of the three variables can be calculated if the other two are given. Thus, with respect to the above question, if the pH and PaCO2 are given (Choice A), the HCO3- can be calculated and with these three data points the acid-base status of the patient can be determined. (Choices B, C, D and E) The PO2 gives information about a patient's oxygenation status. The urinary pH, which can vary widely between 4.5 and 8, can give information about renal handling of acids and bases, but it can not be used to determine a patient's serum acid-base status with the same clarity that pH, HCO3- and PaCO2 can. Educational objective: In any patient, the pH and PaCO2 are the two lab values that provide the best picture of acid-base status; the HCO3- can be calculated from these values using the Henderson-Hasselbalch equation.