Metacresol¶
Overview¶
| Field | Detail |
|---|---|
| Drug class | Preservative (phenolic compound) |
| Route | N/A (excipient/preservative testing) |
| Stock vial | 1034 mg/mL (pure metacresol) |
| Reconstituted conc. | 3 mg/mL (dilute 0.29 mL metacresol in 100 mL NS) |
| Diluent | 100 mL N/S 0.9% |
| Storage | Room temp; Protect from light |
Note
Metacresol is a preservative found in insulin formulations. Testing is performed when investigating insulin allergy where the preservative may be the culprit.
Preparation¶
Before you start
Check the expiry date and vial integrity before opening. Label every syringe immediately after drawing up — never leave unlabelled syringes on the tray. Metacresol is a concentrated phenolic compound — handle with care and use appropriate PPE.
Equipment needed¶
- 1 × Metacresol 1034 mg/mL stock
- Normal saline (NS) 0.9% — 100 mL for initial dilution and IDT dilutions
- 1 mL syringes × 3 (for test doses/dilutions)
- 10 mL syringe × 1 (for initial dilution)
- 25 G needles
Step 1 — Prepare the SPT solution (3 mg/mL — diluted)¶
- Draw 0.29 mL of metacresol stock (1034 mg/mL) into a 10 mL syringe.
- Add 100 mL NS. Mix thoroughly. (This is approximately 3 mg/mL).
- Draw up 0.5–1 mL of this solution into a 1 mL syringe.
- Label: "Metacresol SPT 3 mg/mL".
Step 2 — Prepare IDT dilutions¶
Prepare in this order, using a fresh syringe for each step.
- IDT 0.15 mg/mL (1:20):
- Draw 0.1 mL of the SPT solution (3 mg/mL) into a 1 mL syringe. Add 1.9 mL NS. Mix. (This is 0.15 mg/mL — 1:20).
- Label: "Metacresol IDT 0.15 mg/mL".
Skin prick test (SPT)¶
| Reagent | Concentration |
|---|---|
| Test solution | 3 mg/mL (diluted) |
| Positive control | Histamine 10 mg/mL |
| Negative control | Normal saline |
Interpretation: Wheal ≥3 mm vs negative control = positive.
Intradermal test (IDT)¶
Dilution series¶
| Step | Dilution | Concentration | Preparation |
|---|---|---|---|
| 1 | 1:20 | 0.15 mg/mL | 0.1 mL of 3 mg/mL + 1.9 mL NS |
Inject: 0.05 mL intradermal. Read at 15–20 minutes.