A practical reference for nurses, pharmacists, students and prescribers: buffering local anaesthetics, choosing tubes and timing for drug levels, converting levothyroxine between oral and IV, simple compounding dilutions, syringe compatibility and the difference between irrigation and injection solutions. Local policies and product information always take precedence.
Professional reference: this guide is written for health professionals and students. It summarises commonly used practice and published information; your organisation’s protocols, the product information and your pharmacy department take precedence. Patients should not prepare or adjust injectable medicines themselves.
Quick reference
- Buffered lidocaine: 1 mL sodium bicarbonate 8.4% per 10 mL lidocaine 1% or 2% (with or without adrenaline).
- Levothyroxine PO → IV: IV dose is usually 50–75% of the oral dose, given once daily.
- Nitroglycerin 0.2% ointment: 1 part 2% ointment + 9 parts white soft paraffin, mixed by geometric dilution.
Buffering local anaesthetics
Commercial lidocaine is acidic (pH around 6.5, and lower in solutions with adrenaline, which are made more acidic to stabilise the adrenaline). Acidity is a major cause of the sting on injection, and it also slows onset because less of the drug is in the un-ionised form that crosses nerve membranes. Adding sodium bicarbonate raises the pH towards physiological levels. A meta-analysis of randomised trials found that buffering lidocaine with bicarbonate significantly reduces the pain of injection.
| Anaesthetic | Bicarbonate 8.4% to add | Notes |
|---|---|---|
| Lidocaine 1% or 2% | 1 mL per 10 mL | The standard 10:1 ratio |
| Lidocaine with adrenaline | 1 mL per 10 mL | Adrenaline degrades faster once buffered; prepare close to use |
| Bupivacaine 0.25–0.5% | About 0.1 mL per 10 mL | Precipitates easily; many units don’t buffer it |
| Ropivacaine | Not usually buffered | Precipitates above pH 6 |
Step-by-step preparation, labelling and stability are in how to buffer lidocaine with sodium bicarbonate.
Therapeutic drug monitoring: tubes and timing
For drug levels, timing matters more than anything else: a sample drawn at the wrong time relative to the dose is uninterpretable. Most levels are taken as a trough, immediately before the next dose, once the patient is at steady state (usually after about 4–5 half-lives on a stable dose). Tube type is set by the laboratory and its analyser. Serum tubes without gel (red top) are the traditional choice because gel separators can absorb some drugs, but many labs have validated gel or plasma tubes. Always check your lab’s handbook.
| Drug | Usual sample | Timing |
|---|---|---|
| Valproate (Depakote, Epilim) | Serum (red, or lab-approved gold/SST); some labs accept heparin plasma | Trough, before the morning dose |
| Lithium | Serum; never a lithium-heparin tube | 12 hours after the last dose |
| Digoxin | Serum or heparin plasma | At least 6 hours after the dose (8–12 hours preferred) |
| Phenytoin, carbamazepine | Serum (red preferred) | Trough |
| Vancomycin | Serum or plasma per lab | Trough before the 4th dose, or two levels for AUC-guided dosing |
| Gentamicin, tobramycin | Serum or plasma per lab | Depends on regimen (extended-interval: a single timed level) |
| Tacrolimus, ciclosporin | Whole blood EDTA (lavender/purple) | Trough (12 h after twice-daily dose) |
More detail on valproate and other levels: what colour tube for a Depakote level.
Levothyroxine: oral to IV and back
Oral levothyroxine is roughly 60–80% absorbed, so the intravenous dose that gives the same exposure is lower. US prescribing information for levothyroxine injection recommends an IV dose of about 50–75% of the oral dose, given once daily, with monitoring of clinical status and thyroid function. Because levothyroxine has a half-life of about 7 days, missing oral doses for a few days (for example while nil by mouth) usually doesn’t need IV replacement; IV is used when oral intake is expected to be interrupted for longer, or in myxoedema coma, which uses separate loading regimens. Conversion table and worked example: levothyroxine PO to IV conversion.
Simple compounding: dilutions
Many pharmacy preparations are dilutions of a commercial product. The core formula is C1 × V1 = C2 × V2 (or with weights for semi-solids): the amount of drug stays constant, only the vehicle changes.
| Target | Start with | Ratio | Example for 30 g |
|---|---|---|---|
| Nitroglycerin 0.2% ointment | Nitroglycerin 2% ointment | 1 : 9 | 3 g of 2% + 27 g white soft paraffin |
| Nitroglycerin 0.4% ointment | Nitroglycerin 2% ointment | 1 : 4 | 6 g of 2% + 24 g white soft paraffin |
| Hydrocortisone 0.5% cream | Hydrocortisone 1% cream | 1 : 1 | 15 g of 1% + 15 g compatible base |
Use geometric dilution: mix the active portion with an equal amount of base until uniform, then keep doubling. Label with ingredients, strength, date and a beyond-use date that complies with your standards (for example USP <795> in the US). Worked method and patient counselling: nitroglycerin 0.2% ointment compounding formula.
Mixing medicines in one syringe
Combining two injectable drugs in one syringe reduces the number of injections, but it is only acceptable when compatibility data exist for those products, concentrations and the time between drawing up and giving. Incompatibility can be visible (cloudiness, precipitate, colour change) or invisible (loss of potency). Reliable sources include Trissel’s Handbook on Injectable Drugs, the King Guide to Parenteral Admixtures, the Australian Injectable Drugs Handbook and hospital pharmacy services.
- Mix only immediate-release aqueous formulations; never oily depot products such as haloperidol decanoate.
- Inspect against light and dark backgrounds before giving.
- Draw up immediately before administration and label the syringe with both drugs and doses.
- Follow local policy on whether combined syringes are allowed for the route (IM, SC, IV).
A common example in acute agitation is covered in haloperidol and lorazepam in one syringe.
Irrigation vs injection solutions
Sodium chloride 0.9% is made as two separate product types. Injection/infusion grade is sterile, non-pyrogenic and in containers designed for administration sets. Irrigation grade is sterile but supplied in pour bottles or containers not designed for infusion, and it is labelled “not for injection”. They must not be interchanged, even though the concentration is the same. See sodium chloride irrigation vs saline for injection.
Articles in this guide
Each article answers one specific question in depth.
- How to Buffer Lidocaine with Sodium BicarbonateMix 1 mL 8.4% sodium bicarbonate with 10 mL lidocaine to cut injection pain. Ratios, with adrenaline, precipitation, stability and labelling.
- What Colour Tube for a Depakote (Valproate) Level?Valproic acid levels are usually drawn in a red-top (no gel) or lab-approved serum tube as a trough. Tube guide for common drug levels and timing.
- Levothyroxine PO to IV Dose ConversionIV levothyroxine is usually 50–75% of the oral dose. Conversion table, when to switch back, monitoring and a worked example.
- Nitroglycerin 0.2% Ointment: Compounding FormulaDilute 2% nitroglycerin ointment 1:9 with white soft paraffin to make 0.2%. Worked quantities, mixing, labelling, beyond-use date and patient advice.
- Can Haloperidol and Lorazepam Be Mixed in One Syringe?Haloperidol lactate and lorazepam are often drawn into one syringe for IM use. Compatibility, which haloperidol form, volume and monitoring.
- Sodium Chloride Irrigation vs Saline for InjectionBoth are 0.9% sodium chloride, but irrigation solution is not made for injection. Differences in sterility, labelling, containers and safe use.
Frequently asked questions
Can you buy lidocaine injection over the counter?
No. Injectable lidocaine is prescription-only in Australia, the UK and the US. Topical lidocaine products (creams, patches, sprays up to certain strengths) are available over the counter.
What colour tube is used for a vancomycin trough?
It depends on the laboratory: many use serum (red or gold) or lithium-heparin plasma (green). The timing (trough before a dose, or two timed levels for AUC dosing) matters more. Check your lab handbook.
Why is some IV medicine given slowly?
Rapid injection of some drugs causes high peak levels that lead to side effects: low blood pressure, heart rhythm problems, flushing (“red man” reaction with vancomycin), or vein irritation. Product information gives a minimum injection time or infusion rate, for example IV pantoprazole over at least 2 minutes.
How should a Medrol dose pack be used for poison ivy?
The standard 6-day methylprednisolone pack tapers from 24 mg to 4 mg. Dermatologists often prefer a longer prednisone course (around 2 weeks) for severe poison ivy because short tapers can let the rash rebound. Follow the prescriber’s plan.
Sources and further reading
- Australian Injectable Drugs Handbook (SHPA)
- Specialist Pharmacy Service (NHS) – Medicines advice
- DailyMed – Levothyroxine sodium for injection, prescribing information
- Hanna MN et al. – Efficacy of bicarbonate in decreasing pain on intradermal injection of local anesthetics: a meta-analysis, Reg Anesth Pain Med 2009
- USP – Compounding standards (General Chapter 795)
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