Pharmacology
Infections
Regarding trimethoprim, which of the following statements is INCORRECT:
Answer:
The dose of trimethoprim should be reduced in severe renal impairment as the drug is predominantly excreted by the kidney.Trimethoprim
Pharmacology / Infections
Last Updated: 26th March 2019
Mechanism of Action
Trimethoprim inhibits bacterial metabolism. It is well absorbed orally and is effective in most patients with simple lower urinary tract infections.
A combination of trimethoprim and sulfamethoxazole (co-trimoxazole) may produce a synergistic action and increased activity against certain bacteria. Co-trimoxazole has an important use in the treatment of Pneumocystis jiroveci pneumonia.
Contraindications
Trimethoprim is contraindicated in blood dyscrasias and should be used with caution in the elderly and neonates as it predisposes to folate deficiency. The risk of folate deficiency is also increased when trimethoprim is combined with certain drugs e.g. methotrexate.
There is a teratogenic risk in first trimester (folate antagonist) and manufacturers advise to avoid during pregnancy. However the NICE CKS guidelines state that trimethoprim can be used during pregnancy (although it should not be used first-line when other alternatives are available); if trimethoprim is used during the first trimester, it should be co-prescribed with folic acid 5 mg daily.
The dose should be reduced in severe renal impairment as the drug is predominantly excreted by the kidney.
Side Effects
The most common side effects are allergic reactions and skin rashes. Much less common are more serious reactions such as Stevens-Johnsons syndrome and various blood dyscrasias.
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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 |
