Which long-acting insulins cannot be mixed?

• A. Glargine and Detemir
• B. Humalog and Lantus
• C. NPH and Regular
• D. Aspart and Glargine

Answer: (A)

Long-acting insulins should never be mixed with other insulins in the same syringe because they are formulated to provide a steady, prolonged effect and mixing can alter when and how they act, leading to unpredictable glucose control. Glargine and detemir are both long-acting insulins, so combining them in one syringe is not appropriate. If additional insulin coverage is needed, it should be given with a separate syringe or at a separate time, not mixed together. The other options include rapid-acting insulin paired with a long-acting insulin or non-long-acting insulins; those pairings reinforce the general rule that long-acting insulin should not be mixed with others in the same syringe, but the question specifically demonstrates two long-acting insulins that cannot be mixed.