Other Values

Overview

Blood gas analysers contain instruments to rapidly measure a selection of blood constituents.

It is important to note that the electrolyte and metabolite values may not be as accurate as lab-based analysis, and should therefore be viewed as an approximation when interpreting.

Below are listed some of the values you might find on a blood gas analysis. This is not an exhaustive or detailed summary - see the Resources section for further reading on these values.

Electrolyte Values

Potassium (K⁺): Acid-base disorders alter potassium transport, so it is likely to rise in acidosis and fall in alkalosis.

Sodium (Na⁺): Necessary for calculating the anion gap, and is often deranged in conditions that result in acid-base disorders.

Calcium (Ca⁺) + corrected calcium: Acid-base disorders alter calcium binding, so it is likely to rise in acidosis and fall in alkalosis.

Chloride (Cl^-): Necessary for calculating the anion gap, and identifying hyperchloraemic metabolic acidosis.

Metabolite Values

Glucose (Glu): Useful when considering diabetic ketoacidosis as a differential diagnosis.

Lactate (Lac): Elevated lactate is an indication of tissue hypoxia, and can result in a lactic (metabolic) acidosis.

Oxygen Status

Haemoglobin (ctHb): Concentration of total haemoglobin in the ABG sample, as measured by spectrophotometry.

Oxygen saturation (sO₂): The percentage of haemoglobin fully saturated with oxygen. May not take into account haemoglobin species that cannot carry oxygen (e.g. carboxyhaemoglobin).

Oxygen tension at 50% saturation (p50_c): Calculated figure of the partial pressure of oxygen required to achieve 50% haemoglobin saturation. Normal value: 24-28 mmHg.

Oxygen tension based on alveolar-arterial gradient (pO₂(a/A)_e): A complicated topic that goes well beyond the scope of this tutorial.

Methaemoglobin (FMetHb): A form of haemoglobin that cannot bind oxygen. Methaemoglobinaemia (increased methaemoglobin in the blood) can be inherited (e.g. cytochrome-b₅ reductase deficiency) or acquired (e.g. local anaesthetic toxicity).

Carboxyhaemoglobin spectrophotometry (FCOHb): Carboxyhaemoglobin forms when haemoglobin is exposed to carbon monoxide, which has a much greater affinity than oxygen. Useful when diagnosing carbon monoxide poisoning.

Ideal p50 at standard conditions (p50(st)_c): Ideal value for p50 under normalised conditions. Differs from p50_c by ignoring the patients current condition (which may significantly alter p50_c). Normal value: 24-28 mmHg.

Measurement/estimation of shunt (FShunt_e): Calculated figure which helps determine if an intrapulmonary shunt is present (i.e. blood passing through lung tissue without taking part in gas exchange).

Fraction of oxygenated haemoglobin (FO₂Hb): Percentage of total haemoglobin saturated with oxygen, including haemoglobin species which cannot carry oxygen. Differs from sO₂ which tends to ignore haemoglobin that cannot carry oxygen (e.g. methaemoglobin).

Haematocrit (Hct_c): Calculated volume of red blood cells as a fraction of total blood volume. Normal values: 0.40-0.54 (men), 0.36-0.48 (women).