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Lessons · Medicine foundations · a reference range, and what within it means

A reference range is two numbers and a rule

A reference range is a lower limit and an upper limit, both given to you; a value is within the range when it sits at or between them, and how far outside it is, if it is, is a subtraction.

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 printed report comes back with a small arrow beside one line, and the arrow feels like a verdict. It is not. It means only that the value sits outside the range that laboratory used, and that range is printed on the same sheet, two columns to the right. Reading the range is the skill; the arrow is somebody else's arithmetic.

How to think about it

Write the two limits down before you look at the value again. Compare: below the lower limit, between them, or above the upper limit. If it is outside, subtract the nearer limit from the value, and say which limit you used. The width of the range, upper minus lower, is worth a line too, because it tells you whether being 0.3 out is a long way or nothing at all.

Worked example

Range given by the problem: 3.5 to 5.0 mmol/L
Two numbers, from the question. Nothing is remembered here.
Value 3.2 mmol/L, which is below 3.5, so it is outside the range
A comparison, stated in words before any subtraction.
3.5 − 3.2 = 0.3 mmol/L below the lower limit
How far out, and which limit it is measured from.
Width of the range: 5.0 − 3.5 = 1.5 mmol/L
Being 0.3 out of a range 1.5 wide is a fifth of the whole range.

Your turn

A problem gives a sodium reference range of 135 to 145 mmol/L and a value of 149 mmol/L. Write the line that gives how far above the upper limit it sits.

149 − 145 =  mmol/L

The trap

Measuring from the middle of the range instead of from the nearer limit. A range is not a target with a bullseye; every value between the two limits is inside it, and the distance that means anything is the distance to the limit you crossed.

Practise a reference range, and what within it means on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.