What is the difference between millimoles (mmol) and milligrams (mg) in medical contexts?

Understanding mmol vs mg in Medical Contexts

Millimoles (mmol) and milligrams (mg) are fundamentally different measurement systems: mg measures mass (weight) of a substance, while mmol measures the number of molecules present, making mmol more scientifically precise for understanding biological activity but mg more practical for clinical dosing.

Core Distinction

The fundamental difference lies in what each unit measures:

• Milligrams (mg): A mass-based unit measuring the actual weight of a substance 1
• Millimoles (mmol): A molar unit measuring the number of molecules, directly related to molecular quantity 1

One mole equals 6.022 × 10²³ molecules (Avogadro's number), so mmol reflects molecular count rather than weight 1.

Conversion Between Units

To convert between these units, you must know the molecular weight (molar mass) of the specific substance:

• mg to mmol: Divide mg by the molecular weight 2
• mmol to mg: Multiply mmol by the molecular weight 2

Common Clinical Examples

Glucose conversion (molecular weight = 18):

• To convert mg/dL to mmol/L: divide by 18 2, 3
• Example: 126 mg/dL ÷ 18 = 7.0 mmol/L 3
• Example: 180 mg/dL ÷ 18 = 10.0 mmol/L 3

Lipid panel conversions 2:

• Total cholesterol, LDL-C, HDL-C, non-HDL-C: divide mg/dL by 38.6 to get mmol/L
• Triglycerides: divide mg/dL by 88.6 to get mmol/L

Electrolyte examples 2:

• Vitamin A (retinol): 1 mg/dL = 0.0357 mmol/L
• Calcium: varies by salt form used
• Phosphorus: varies by salt form used
• Magnesium: 1 mmol = approximately 24 mg elemental magnesium

Clinical Application and Regional Differences

Geographic variation in preferred units 3:

• North America predominantly uses mg/dL for glucose and most drugs 3, 4
• Most other countries use mmol/L for glucose and electrolytes 3
• This creates potential for confusion when interpreting international research 3

Why mass units (mg) dominate drug dosing 4:

• Drugs are prescribed, manufactured, and dispensed by weight (mg) 4
• Converting to molar dosing would cause massive disruption and patient safety risks 4
• Mass units are more practical for pharmaceutical manufacturing and clinical use 4
• Changing to molar units would be expensive without clear benefit to patients 4

When molar units (mmol) are preferred 1:

• Electrolyte management (sodium, potassium, calcium, phosphorus, magnesium) 2
• Comparing biological activity of different molecular forms of the same substance 1
• Parenteral nutrition formulations where precipitation risk depends on molar ratios 2
• Understanding stoichiometric relationships in metabolism 1

Critical Pitfalls to Avoid

Common errors that can harm patients 3:

• Confusing the conversion factor: Multiplying by 18 instead of dividing when converting glucose from mg/dL to mmol/L 3
• Misinterpreting international literature: Failing to check which unit is being reported 3
• Ignoring sample type differences: Plasma vs whole blood may have different reference ranges even in the same units 3
• Assuming universal conversion factors: Each substance has its own molecular weight; glucose conversion (÷18) does NOT apply to other substances 2

Specific clinical scenarios requiring attention:

• Neonatal care: Reference ranges for electrolytes differ significantly in newborns; adult reference values in mmol/L may lead to underdiagnosis of deficiency 2
• Parenteral nutrition: Calcium-phosphorus ratios must be calculated in mmol to prevent precipitation; using mg can lead to dangerous errors 2
• Renal disease: Magnesium and phosphorus handling changes dramatically; molar measurements better reflect filtered load 2

Practical Clinical Algorithm

When reviewing any laboratory value or research data:

1. Identify the unit being used (mg/dL, mg/L, mmol/L, etc.) 3
2. Know your local laboratory's reference ranges and units 3
3. For international research, verify the conversion factor for that specific substance 2, 3
4. For electrolytes and metabolic parameters, think in mmol for physiologic understanding 2
5. For drug dosing, always use mass units (mg) as prescribed 4
6. When calculating parenteral nutrition, use mmol for mineral ratios to prevent precipitation 2

The key principle: Mass (mg) tells you how much substance by weight; moles (mmol) tells you how many molecules are present, which determines biological activity 1.