Haemoximetry

Keywords: Oximetry

Description:

Rarely performed.

See also Carboxyhaemoglobin, Methaemoglobin, Sulphaemoglobin.

Specimen:

1 mL arterial blood collected anaerobically in a heparinised syringe and transported as soon as possible to the laboratory with the needle removed and the syringe capped.

Also in vivo test (pulse oximetry).

Method:

Spectrophotometry: assay of oxyhaemoglobin, methaemoglobin, carboxyhaemoglobin, sulphaemoglobin, total haemoglobin.

Derived parameters include Hb saturation and P50 O2 (requires measurement of arterial pO2 using oxygen selective electrode).

Oxygen saturation can be measured in vivo using a pulse oximeter.

Reference Interval:

Hb oxygen saturation: 94.5-98.2 %

Methaemoglobin: < 1.0 %

Carboxyhaemoglobin: < 1.5 % (non-smoker)

Sulphaemoglobin: not detected

P50 O2: 3.4-3.8 kPa (25-29 mmHg).

Application:
  1. Investigation of efficiency of haemoglobin oxygenation by the lung (Hb saturation).

  2. Measurement of non-oxygen-carrying blood pigments (Carboxyhaemoglobin, Methaemoglobin, Sulphaemoglobin).

  3. Investigation of patients with likely abnormalities of oxygen carriage and release eg, acidosis, alkalosis, hypoxaemia.

Interpretation:

Decreased Hb saturation indicates impaired ventilation, diffusion, ventilation/perfusion ratio or the presence of a right to left shunt.

The P50 O2 is defined as the pO2 at which haemoglobin is 50% saturated. Decreased P50 O2 ('shift to the left') indicates increased oxygen affinity of blood (eg, decreased 2,3 diphosphoglycerate levels, alkalosis) and hence impaired oxygen delivery to tissues.

Other derived parameters quantitating the availability of oxygen to tissues have been proposed, but none has achieved general acceptance.

Reference:

Siggaard-Andersen O et al. Crit Care Med. 1995; 23: 1284-1293.