The correct order
When regular insulin and neutral protamine Hagedorn (NPH) insulin are mixed in one syringe, the principle is clear before cloudy. Air is injected into the NPH (cloudy) vial first without touching the insulin, then into the regular (clear) vial, and the regular insulin is withdrawn before the NPH. The correct sequence is 3, 1, 4, 2: inject 14 units of air into NPH, inject 6 units of air into regular, withdraw 6 units of regular, then withdraw 14 units of NPH.
Why the order matters
Regular insulin is a clear, short-acting solution. NPH is a cloudy, intermediate-acting suspension containing protamine, which delays absorption. If NPH enters the regular insulin vial, even in small amounts, the protamine can bind some of the regular insulin and alter its action for every dose drawn from that vial afterward. Drawing clear before cloudy keeps NPH from contaminating the regular insulin vial. Injecting air into the NPH vial first, while the syringe is still empty, means no insulin can be pushed into it. Air is then injected into the regular vial, and the regular dose is drawn while the syringe contains only air and regular insulin.
| Step | Action | Purpose |
|---|
| 1st | Inject 14 units of air into NPH | Equalizes pressure; syringe is empty |
| 2nd | Inject 6 units of air into regular | Equalizes pressure in the clear vial |
| 3rd | Withdraw 6 units of regular | Clear insulin drawn first |
| 4th | Withdraw 14 units of NPH | Total 20 units in syringe |
Why the other sequences are wrong
Sequence 1, 3, 2, 4 withdraws NPH before regular, so the needle enters the regular vial with NPH in the syringe. Sequence 3, 2, 1, 4 withdraws NPH and then enters the regular vial, again risking contamination. Sequence 1, 4, 3, 2 injects air into NPH with regular insulin already in the syringe, which can push regular insulin into the NPH vial.
Safety checks
Before preparing the dose, the NPH vial is gently rolled, not shaken, to resuspend it. The nurse verifies the order, checks the total of 20 units, and, if any excess is drawn from the second vial, discards the syringe and starts again rather than returning a mixture to the vial. The mixed insulin is given promptly.
Exam takeaway
Key point Air into cloudy, air into clear, draw clear, draw cloudy. Clear before cloudy protects the regular insulin vial from contamination.