Anion gap

Is calculated as: ([Na+] + [K+]) − ([Cl−] + [HCO3−]), all units in mmol/l (Normal: 16 ± 4 mmol/l).

Increases in anion gap seen in:

1. ▪ Diabetic ketoacidosis

2. ▪ Uraemic acidosis

3. ▪ Drug ingestion (e.g. salicylates)

4. ▪ Lactic acidosis

5. ▪ Hypokalaemia

6. ▪ Hypocalcaemia

7. ▪ Hypomagnesaemia

8. ▪ Hyperalbuminaemia

9. ▪ Laboratory error.

A decreased anion gap is less frequent but can be seen in:

1. ▪ Hypoalbuminaemia

2. ▪ Increased immunoglobulins (e.g. myeloma)

3. ▪ Hyperkalaemia

4. ▪ Hypercalcaemia

5. ▪ Hypermagnesaemia

6. ▪ Lithium therapy.

Body mass index (BMI)

BMI = Weight (kg)/Height (m)2.

1. ▪ A BMI of ≤ 20 means the patient is underweight.

2. ▪ 20–25 is desirable.

3. ▪ 25–30 is overweight.

4. ▪ > 30 is obese.

Body surface area (BSA)

1. ▪ BSA (m2) = 0.20247 × Height (m)0.725 × Weight (kg) – Dubois and Dubois method.

2. ▪ BSA (m2) = (Height (cm) ×Weight (kg)/3600) – Mosteller method.

Cardiac output (CO) (Fick method)

The cardiac output can be estimated by dividing oxygen consumption by the difference in oxygen content between arterial and venous blood. The difference between the arterial and mixed venous blood oxygen concentration correlates with oxygen uptake per unit of blood as it flows through the lungs (Fick principle). The method is cumbersome due to the need to collect expired air and arterial blood gases.

where 1.34 ml per gram is the amount of oxygen that a gram of haemoglobin can carry if 100% saturated.