Mixing & dilution guide¶
General principles for drug preparation for allergy skin testing at RPAH Clinical Immunology & Allergy.
Diluents¶
| Diluent | Abbreviation | When to use |
|---|---|---|
| 0.9% Sodium Chloride | NS / N/S | Default diluent for most drugs |
| Water for Injection | WFI | When specified (e.g. Benzylpenicillin, most powder vials) |
| Saline in phosphate buffer | — | Penicillin determinants (PPL, MD) only |
| Nil | — | Ready-to-use solutions (e.g. Rocuronium, Propofol) |
Serial dilution method¶
To make a 1:10 dilution: take 1 mL of solution + 9 mL diluent.
To make a 1:100 dilution: two options: - Direct: 0.1 mL of stock + 9.9 mL diluent - Serial: 1 mL of 1:10 dilution + 9 mL diluent
To make a 1:1,000 dilution: 1 mL of 1:100 + 9 mL diluent (and so on).
Label every syringe immediately after preparation
Include: drug name, concentration, date/time prepared, preparer initials.
IDT volumes and technique¶
- Standard IDT volume: 0.05 mL (50 µL)
- Use a 1 mL tuberculin syringe with a 26–27 g needle
- Inject intradermally — a visible bleb of 3–5 mm should form
- If no bleb forms, the injection was subcutaneous — discard and repeat at a new site
SPT technique¶
- Apply a small drop of test solution to the volar forearm
- Prick through the drop at 90° with a sterile lancet
- Blot excess solution after 1 min — do not wipe
- Positive control (histamine 10 mg/mL) and negative control (NS) must be included
- Space test sites at least 3 cm apart
- Read at 15–20 min
Positive SPT: Wheal ≥ 3 mm greater than the negative control
Powder vial reconstitution reference¶
| Drug | Vial size | Add diluent | Reconstituted concentration |
|---|---|---|---|
| Amoxicillin | 1 g | 9.2 mL WFI | 100 mg/mL |
| Ampicillin | 1 g | 9.2 mL WFI | 100 mg/mL |
| Cefazolin | 1 g | 10 mL WFI | 100 mg/mL |
| Ceftazidime | 2 g | 20 mL WFI | 100 mg/mL |
| Cefotaxime | 1 g | 10 mL WFI | 100 mg/mL |
| Ceftriaxone | 1 g | 10 mL WFI | 100 mg/mL |
| Cefepime | 1 g | 10 mL WFI | 100 mg/mL |
| Benzylpenicillin | 600 mg | See Benzylpenicillin page | 6 mg/mL (neat for testing) |
| Vecuronium | 10 mg | 2.5 mL WFI | 4 mg/mL |
| Remifentanil | 1 mg | 20 mL WFI | 0.05 mg/mL |
| Pantoprazole | 40 mg | 10 mL NS | 40 mg/mL |
Waste and disposal¶
- Discard all prepared solutions at end of testing session
- Do not store reconstituted solutions beyond the times specified on individual drug pages
- Dispose of sharps immediately into sharps container
Pre-testing checklist¶
- Written consent obtained and documented
- Baseline vitals recorded (BP, HR, SpO₂)
- Adrenaline 0.5 mg/mL (1:1000) drawn, labelled, and within reach
- IV access established (for IDT and all challenges)
- Antihistamines withheld ≥ 72 h (check with AMO if unclear)
- Beta-blockers noted (may blunt anaphylaxis treatment response — inform AMO)
- Resuscitation equipment checked and available