This section describes the laboratory tests used in pH abnormalities, which, incidentally, are often called “acid-base problems” because of the importance of the bicarbonate and carbonic acid changes involved.

Carbon dioxide combining power. Venous blood is drawn aerobically with an ordinary syringe and the serum is then equilibrated to normal alveolar levels of 40 mm Hg by the technician blowing his or her own alveolar air into the specimen through a tube arrangement. This maneuver adjusts the amount of dissolved CO2 to the normal amounts found in normal arterial blood. Bicarbonate of the serum is then converted to CO2 by acid hydrolysis in a vacuum, and the released gas is measured. The released CO2 thus consists of the dissolved CO2 of the specimen already present plus the converted HCO–3 (and thus the denominator plus the numerator of the Henderson-Hasselbalch equation). Subtraction of the known amount of dissolved CO2 and H2CO3 in normal blood from this measurement gives what is essentially a value for serum HCO–3 alone (called the “combining power,” since HCO–3 combines with H+). The inaccuracy that may be caused by these manipulations should be obvious.

Carbon dioxide content. Total CO2 content is determined from heparinized arterial or venous blood drawn anaerobically. This may be done in a vacuum tube or a syringe that is quickly capped. (Mineral oil is not satisfactory for sealing.) The blood is centrifuged and the plasma removed. At this point, all the CO2 present is still at the same CO2 tension or partial pressure of dissolved gas that the patient possessed. Next, the plasma is analyzed for CO2 by a method that converts HCO–3 and H2CO3 to the gas form. Thus CO2 content measures the sum of HCO–3, H2CO3, and dissolved CO2. Since the amount of dissolved CO2 and H2CO3 in blood is very small, normal values for CO2 content are quite close to those of the CO2 combining power (which measures only HCO–3). Since the specimen has been drawn and processed with little or no contact with outside air, the result is obviously more accurate than that obtained from the CO2 combining power.

Serum bicarbonate. An order for serum CO2, serum HCO–3, or venous CO2 will usually result in serum being obtained from venous blood drawn aerobically and assayed for HCO–3 without equilibration. This technique is used in most automated equipment that assays “CO2” (really, bicarbonate) in addition to performing other tests, and is also popular in many laboratories using manual methods since the patients usually have other test orders that require serum. It is somewhat less accurate than CO2 combining power. The serum is frequently exposed to air for relatively long periods of time. Only relatively large changes in CO2 or HCO–3 will be detected. Underfilling of specimen collection tubes to only one third of capacity significantly decreases bicarbonate values.

Partial pressure of carbon dioxide (PCO2).

PCO2 is the partial pressure of CO2 gas in plasma or serum (in mm of Hg or in Torr); this is proportional to the amount of dissolved CO2 (concentration of CO2). Since most of the denominator of the Henderson-Hasselbalch equation represents dissolved CO2, and since PCO2 is proportional to the amount (concentration) of dissolved CO2, PCO2 is therefore proportional to the denominator of the Henderson-Hasselbalch equation and may be used as a substitute for the denominator. In practice, a small amount of whole blood (plasma can be used) collected anaerobically is analyzed for PCO2 by direct measurement using a PCO2 electrode. The HCO–3 may then be calculated (from the Henderson-Hasselbalch equation), or the PCO2 value itself may be used in conjunction with pH to differentiate acid-base abnormalities. PCO2 determined by electrode is without question the method of choice for acid-base problems. The pH is usually measured at the same time on the same specimen.

pH measurement. pH determination originally involved measuring the difference in electric charge between two electrodes placed in a solution (e.g., plasma or whole blood). Current equipment uses a single direct-reading pH electrode, which makes pH determination very simple and reliable and enables pH determination to be a routine part of blood gas measurement. On the technical side, it should be noted that at room temperature, plasma pH decreases at the rate of about 0.015 pH unit every 30 minutes. Unless measurement is done within 30 minutes of drawing, the blood should be refrigerated immediately; it can then be kept up to 4 hours.

There is not a universally accepted single reference range for arterial or venous acid-base parameters.