Skip to content

Benzylpenicillin

Overview

Field Detail
Drug class Penicillin (natural beta-lactam)
Route IV
Stock vial 600 mg powder for injection
Final SPT/IDT concentration 6 mg/mL (via multi-step dilution — see Preparation)
Diluent Normal saline (0.9% sodium chloride)
Storage Use within 24 hours at 2–8°C

Preparation

Multi-step reconstitution required

Benzylpenicillin cannot be used directly — it requires a specific multi-step dilution to reach the working concentration of 6 mg/mL. Follow the steps below carefully.

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.

Equipment needed

  • 1 × Benzylpenicillin 600 mg powder vial
  • Normal saline (0.9% sodium chloride) — at least 25 mL
  • Syringes: 1 × 2 mL, 2 × 10 mL, 4 × 1 mL
  • 21 G needles (for vial entry and transfers)
  • 25 G needles (for drawing up final dilutions)
  • Labels or marker pen

Step 1 — Initial reconstitution → 300 mg/mL

  1. Draw up 1.6 mL of normal saline into a 2 mL syringe.
  2. Inject into the Benzylpenicillin 600 mg vial. Gently swirl until dissolved.
  3. This gives Benzylpenicillin approx. 300 mg/mL. Label the vial: "BenPen 300 mg/mL".

Step 2 — First dilution → 30 mg/mL

  1. Draw up 1 mL of the 300 mg/mL solution into a clean 10 mL syringe.
  2. Add 9 mL of normal saline. Mix gently.
  3. This gives Benzylpenicillin 30 mg/mL. Label the syringe: "BenPen 30 mg/mL".

Step 3 — Second dilution → 6 mg/mL (SPT/IDT neat)

  1. Draw up 1 mL of the 30 mg/mL solution into a clean 10 mL syringe.
  2. Add 4 mL of normal saline. Mix gently.
  3. This gives Benzylpenicillin 6 mg/mL — this is the neat (undiluted) solution used for SPT.
  4. Label the syringe: "BenPen 6 mg/mL — SPT solution".

Step 4 — Prepare IDT dilutions

Prepare in this order, using a fresh syringe for each step.

  1. IDT 0.6 mg/mL (1:10): Draw 1 mL of the 6 mg/mL solution into a 10 mL syringe. Add 9 mL NS. Mix. Label: "Benzylpenicillin IDT 0.6 mg/mL".
  2. IDT 0.06 mg/mL (1:100): Draw 0.1 mL of the 0.6 mg/mL solution into a 1 mL syringe. Add 0.9 mL NS. Mix. Label: "Benzylpenicillin IDT 0.06 mg/mL".

Skin prick test (SPT)

Reagent Concentration
Test solution Neat (6 mg/mL)
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 1:100 0.06 mg/mL 0.1 mL of 0.6 mg/mL + 0.9 mL NS
2 1:10 0.6 mg/mL 1 mL of 6 mg/mL (from Step 3) + 9 mL NS

Inject: 0.05 mL intradermal. Read at 15–20 minutes.

Notes

  • Unique multi-step reconstitution — follow the mixing guide carefully and in order.
  • Often tested alongside PPL and MD determinants as part of a full penicillin workup.