How do you convert mg/dL to mmol/L for glucose (MW 180.16 g/mol)?

• A. mmol/L = mg/dL × 1.0.
• B. mmol/L = mg/dL × 0.0555.
• C. mmol/L = mg/dL ÷ 0.0555.
• D. mmol/L = mg/dL × 18.0.

Answer: (B)

Knowing how to convert mg/dL to mmol/L hinges on turning a mass concentration into a molar concentration using the substance’s molecular weight. For glucose, with a molecular weight of about 180.16 g/mol, you start by recognizing that 1 mg/dL equals 0.01 g/L. To get moles per liter, divide by the molecular weight to get mol/L, then multiply by 1000 to convert to mmol/L. This gives mmol/L = (mg/dL) × (0.01 / 180.16) × 1000 = (mg/dL) × (10 / 180.16) ≈ mg/dL × 0.0555. Therefore, mmol/L = mg/dL × 0.0555, which is the correct relationship. This is the same as dividing by about 18, since 1/18 ≈ 0.0555. The other formulations either keep the same mg/dL value, multiply by a factor that’s too large, or invert the conversion, which don’t correctly convert to mmol/L for glucose.