Physiology
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Renal
You are calculating the anion gap on a patient with an acid-base disturbance and you find the anion gap to be low. Which of the following is the most likely cause for the low anion gap:
Answer:
A low anion gap is frequently caused by hypoalbuminemia. Albumin is a negatively charged protein and its loss from the serum results in the retention of other negatively charged ions such as chloride and bicarbonate. As bicarbonate and chloride anions are used to calculate the anion gap, there is a subsequent decrease in the gap. The anion gap is sometimes reduced in multiple myeloma, where there is an increase in plasma IgG (paraproteinaemia).Anion Gap
Physiology / Renal / Acid-Base Balance
Last Updated: 21st April 2019
Definition
The anion gap is the difference between the measured cations and the measured anions in plasma. As plasma is always electrically neutral, the anion gap determines the presence of unmeasured anions. These are usually proteins, organic acids, sulphate and phosphate.
The anion gap can be calculated as: ([Na+] + [K+]) - ([Cl-] + [HCO3-])
The normal anion gap is normally between about 6 - 16 mmol/L.
Clinical Relevance
In metabolic acidosis:
- An increased anion gap occurs if a new acid is added to the body. This dissociates producing free H+ (which uses up bicarbonate) and anions (which take the place of the bicarbonate).
- A normal anion gap occurs if there is a simple loss of bicarbonate. This causes a compensatory rise in plasma chloride concentration so the anion gap is normal (thus is sometimes referred to as hyperchloraemic acidosis).
Causes of Increased Anion Gap Metabolic Acidosis
MUDPILES can be used to remember some of the causes of a raised anion gap acidosis:
- Methanol
- Uraemia (in renal failure)
- Diabetic ketoacidosis
- Propylene glycol overdose
- Infection/Iron overdose/Isoniazid/Inborn errors of metabolism
- Lactic acidosis
- Ethylene glycol overdose
- Salicylate overdose
Causes of Normal Anion Gap Metabolic Acidosis
FUSEDCARS can be used to remember some of the causes of a normal anion gap acidosis:
- Fistula (pancreaticoduodenal)
- Ureteroenteric conduit
- Saline administration
- Endocrine (hyperparathyroidism)
- Diarrhoea
- Carbonic anhydrase inhibitors (e.g. acetazolamide)
- Ammonium chloride
- Renal tubular acidosis
- Spironolactone
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
