Guided case: saturation normal, content low

Arterial blood is collected through an arterial line, with air bubbles expelled immediately and analysis completed within 10 minutes. pH is 7.38 with a reference interval of 7.35 to 7.45, PaCO2 is 41 mmHg with interval 35 to 45, and PaO2 is 88 mmHg with interval 80 to 100 on room air. Calculated sO2 and co-oximetry sO2 are both 97%. The curve and gas results do not suggest a shift-driven explanation for low content.

Co-oximetry reports FO2Hb 94%, COHb 2.6%, and MetHb 0.6%. Both fractions are compared with illustrative low-range values; the named laboratory's validated co-oximetry reference intervals, which are method- and population-specific, govern release and interpretation. Dyshemoglobins are not the dominant explanation here. Hemoglobin is 8.1 g/dL. Using the stated formula, CaO2 equals (1.34 × 8.1 × 0.97) + (0.003 × 88), or 10.8 mL O2/dL.

At a comparison hemoglobin of about 15 g/dL with similar saturation, CaO2 would be roughly 20 mL O2/dL. The dissolved term contributes only 0.264 mL O2/dL in this case. The lower content is therefore chiefly a hemoglobin-mass issue, not evidence that PaO2 or saturation is falsely low. A near-normal PaO2 and saturation can coexist with low CaO2 when hemoglobin is low.