Lessons · Medicine foundations · mmol/L and mg/dL

The same blood, two units: mmol/L and mg/dL

One laboratory prints a result in millimoles per litre and another prints the same result in milligrams per decilitre; a single factor, given to you, turns either one into the other.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A paper in front of you talks about a glucose of 5.5 and the textbook beside it talks about 99, and for a while you think they disagree. They do not. Same blood, same person, two units, and until you can move between them every number you read is half a number.

How to think about it

Write the value down with its unit before anything else. Then ask one question: which way am I going? Milligrams per decilitre divided by the factor gives millimoles per litre; millimoles per litre multiplied by the factor gives milligrams per decilitre. The factor belongs to the substance, and the problem always hands it to you.

Worked example

Glucose 90 mg/dL, and the problem gives the factor 18

The factor is part of the question, not something to remember. A different substance has a different factor.

90 / 18 = 5.0 mmol/L

Going to millimoles, so divide. Five millimoles per litre.

Glucose 5.5 mmol/L, the same factor 18

Now the other direction, on a different value.

5.5 × 18 = 99 mg/dL

Going to milligrams per decilitre, so multiply. Ninety-nine.

Your turn

A report gives glucose 126 mg/dL, and the problem gives the factor 18. Write the line that puts it in millimoles per litre.

126 / 18 =  mmol/L

The trap

Borrowing a factor from another substance because it is the one you happen to remember. Glucose, creatinine and cholesterol each have their own, and the one you need is printed in the question you are answering.

Practise mmol/L and mg/dL on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.