NAPLEX Question of the Week: Trading Insulins

Insulins are a common source of medication error/misadventure. Are you up to this week's challenge?
NAPLEX Question of the Week: Trading Insulins

NT is a 66-year-old female that presents to your ambulatory care clinic with a referral for managing her diabetes regimen. NT has a history of type 2 diabetes mellitus for 2 years and COPD. She reports being adherent to her medications, but notes that her morning blood glucose readings have been high and always being thirsty. NT also complains about the price of her insulin is getting high since she is the donut hole.


Current Medications: Levemir FlexPen 18 units SQ BID, Farxiga 5 mg PO QD, Metformin ER 1000 mg PO BID, Trelegy 100/62.5/25 mcg 1 puff QD, Ventolin 90 mcg 2 puffs q4-6 hours PRN

Pertinent Labs:

  • BUN: 15 mg/dL; SCr: 1.0 mg/dL; Na: 138 mEq/L; K 4.4 mEq/L, Cl: 102 mEq/L Mg: 2.0 mg/dL; SBG: 240 mg/dL; AST: 22 U/L; ALT: 28 U/L, A1c: 10.3% (Previous A1c 6 months ago: 12%)
  • Average Fasting BG based on patient log: 250
  • Vitals: BP: 128/78 mmHg; HR: 72; RR: 18; Temp: 99.0F
  • Weight: 79 kg; Height: 66 in

The collaborating physician at your ambulatory clinic would like to transition the patient Humulin N. Based on the patient’s current insulin regimen, what would be the most appropriate Humulin N dosing regimen to convert NT to?

A. Humulin N 24 units SQ QAM and 12 units SQ QHS

B. Humulin N 45 units SQ QHS

C. Humulin N 30 units SQ QAM and 15 units SQ QHS

D. Humulin N 12 units SQ QAM and 12 units SQ QHS

Rationale with explanation:

Brands/Generics covered: Levemir (insulin detemir), Farxiga (dapaglifozin), Trelegy (fluticasone/umeclidinium/vilanterol), Ventolin (albuterol), Humulin N (insulin NPH) 


Insulins are primarily characterized by their duration of action, which range from ultrarapid acting to ultra-long acting. When switching insulins is important to note the duration of action for the insulin product that will be discontinued and the insulin product that patient will begin using.  In this scenario, the patient is currently on Levemir, a long-acting insulin. Based on the patient’s current insulin regimen, there is room to increase her basal dose of Levemir. However, physician would like to switch to insulin NPH, an intermediate acting insulin, to accommodate the patient’s financial concerns.

When switch from a long-acting insulin to intermediate-acting insulin, the total daily dose (TDD) of insulin should be calculated. The TDD can be converted unit-per unit to NPH, or the TDD can be reduced by 20%. The converted dose should be divided into twice daily dosing for insulin NPH.  The dose can be divided evenly (50:50) for morning and evening doses, with evening doses preferably administered at bedtime to reduce risk for nighttime hypoglycemia.  Alternatively, 2/3 of the TDD can be given in the morning and 1/3 of the TDD can be given at be at bedtime.

Answer A is correct. NT’s TDD of Levemir is 36 units. Converting unit-per-unit, NT’s insulin NPH TDD would be 36 units. The dosing scheme for this answer choice uses the 2/3 of TDD in the morning and 1/3 of TDD at bedtime. As mentioned above, a dose decrease of up to 20% could have been considered as well. This patient's A1c is over 10% so reasonable to do a 1:1 conversion. 

Answer B is incorrect. This answer choice would result in a 25% increase in TDD of insulin. While the patient is still experiencing hyperglycemia based on her BG log and current A1c, insulin should only be increased by 10-15% or 2 units every 3 days until fasting BG is at goal (80-130 mg/dL). Due to the pharmacokinetics of NPH and it peaks at 4-6 hours after injection versus the relatively stable concentration of long-acting insulins, there is an increased risk of hypoglycemia with this formulation of insulin. Therefore, increasing the dose this much may result in hypoglycemic episodes, which can be exhibited as sweating, dizziness, confusion, tachycardia, and/or shaking. The patient may need to increase her TDD of insulin, but standard conversions recommend converting unit-per-unit or reducing the TDD of long-acting insulin by 20% rather increasing the TDD immediately. Furthermore twice-daily dosing is the preferred dosing frequency for insulin NPH due to the shorter duration of action compared to long-acting insulins, such as glargine.

Answer C is incorrect. While this answer choice uses the 2/3 QAM and 1/3 QPM dosing scheme, the TDD is 45 units for this conversion. Increasing the TDD of insulin to 45 units is greater than the recommended max increase of 15%. Additionally, the long-acting insulin TDD should be converted unit-per-unit or reduced by 20% when converting to intermediate-acting insulin.

Answer D is incorrect.  Based on the current scenario, this answer choice reduces the TDD of insulin by 33%, which would probably lead to further inadequate glycemic control for NT. 

NAPLEX Competencies Covered:

Area 3 (Develop or Manage Treatment Plans), 3.4 Drug dosing or dosing adjustments, 3.10 Drug pharmacokinetics or pharmacodynamics; Area 4 (Perform Calculations), 4.2 Quantities of drugs to be dispensed or administered, 4.4 Dose conversions

Hopefully this was helpful for you as you prepare for the NAPLEX!

Dr. B

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Go to the profile of angel ng
6 months ago

Answer choice C comes out to 45 units not 50 units as stated in answer key but the reasoning still holds. Good question like many others!

Go to the profile of Christopher M. Bland
6 months ago

Thank you for your input!