Screening for hemoglobin variants with abnormal oxygen affinity.

To the Editor: Rai et al. (1 ) recently described an elegant method for detecting hemoglobin (Hb) variants that cause erythrocytosis, using electrospray mass spectrometry. Many of these variants are not detectable with conventional electrophoretic techniques. The authors also stated that “functional studies of oxygen saturation curves would be the ideal method to screen for Hb variants that cause erythrocytosis, but such methods are not generally available. . . . ” On the contrary, the determination of P50 (partial pressure of O2 at which Hb is half-saturated) is easily done in any laboratory with a blood gas analyzer and a multiwavelength oximeter. The P50, when normalized to pH 7.40 and a Pco2 of 5.33 kPa (40 mmHg), is a direct (inverse) reflection of Hb oxygen affinity. A onepoint method and calculation have been described in an IFCC Guideline (2 ), and these or equivalent calculations have been included in the software of many blood gas analyzers and can be performed automatically. Heparinized venous blood is used. One advantage of electrospray mass spectrometry is that specimens do not need to be analyzed immediately, but can be stored and shipped to a laboratory where the electrospray mass spectrometry method is available. On the other hand, heparinized whole blood specimens for P50 determination are stable for at least 2–3 h at room temperature, and minimal change is observed after 24 h when specimens are held at 4 °C (3 ). We have routinely used P50 measurements with instruments in the laboratory for many years to screen for Hb variants in patients with unexplained anemia or erythrocytosis.