Why the same result looks completely different in another country

Someone comparing their results with a friend abroad, or reading advice written for another country, quickly runs into numbers that seem impossible. A cholesterol of 5.2 and a cholesterol of 200 sound like different worlds. They are the same amount of cholesterol.

The difference is the unit, and understanding it removes a large source of unnecessary alarm.

Two ways of counting the same thing

Most of the world reports many blood tests in millimoles per litre, which counts molecules. The United States and a few other countries report the same tests in milligrams per decilitre, which counts mass.

Because the conversion depends on the molecular weight of whatever is being measured, there is no single factor that converts one system to the other. Glucose, cholesterol and creatinine each have their own.

This is why a number copied from an article written elsewhere cannot be compared directly with a number on a local report, even for the same marker.

The conversions people meet most often

Glucose in millimoles per litre multiplied by about eighteen gives milligrams per decilitre. A fasting glucose of 5.5 is a little under 100 in the other system.

Total cholesterol in millimoles per litre multiplied by about thirty-nine gives milligrams per decilitre. A cholesterol of 5.2 is roughly 200.

Creatinine runs the other way for most people's intuition: milligrams per decilitre multiplied by about eighty-eight gives micromoles per litre. A creatinine of 1.0 is close to 88.

These are the reasons a report should always be read with its own units attached. A bare number carries almost no information.

Units that hide a scaling factor

Cell counts add a second complication. A white cell count may be printed as 6.2 with units of ten to the ninth per litre, or as 6200 per microlitre, or as 6.2 thousand per microlitre. All three describe the same blood.

Platelets are reported the same way, which is why a platelet count can appear as 250 on one report and 250,000 on another without anything having changed.

Where a report prints a count without making the scaling explicit, the reference range beside it usually reveals which convention is in use.

HbA1c, which changed units within living memory

HbA1c was long reported as a percentage and is now widely reported in millimoles per mole. Many countries print both, and a great deal of older material online uses only the percentage.

The two scales are not proportional to each other, so converting by eye does not work. An HbA1c of 6.5 per cent corresponds to about 48 millimoles per mole, and a reader who assumes the numbers are interchangeable will draw the wrong conclusion in either direction.

Practical advice

Read every result together with its unit and the range printed beside it, and treat a number found elsewhere as belonging to a different scale until proven otherwise.

When keeping your own record of results over time, record the unit alongside the value. A list of bare numbers becomes ambiguous surprisingly quickly, particularly if testing moves between providers or countries.

If a result seems wildly out of keeping with everything else on a report, checking the unit is a sensible first move before assuming the value is remarkable.