Hello Gert,

The "normal range" does not exactly refer to "normal" it really refers to this statistical observation of "normal":

For pretty much any number you can measure in people - from blood pressure, to Hb, to ALT - if you measure enough people you get a curve like this, which is called a bell curve. The distribution of values is called the "normal distribution".

You can see that the biggest count of people having that value lies in the middle. As we more to the left (or right) we see less and less values.

Your eyes may have glazed over in maths and statistics when the teacher talked about mean, median, and standard deviation. The mean is the average value and is in the middle of a normal distribution. The median has 1/2 the values above, and 1/2 below. It is the same as the mean in a normal distribution. The standard deviation is interesting in that 95% of all values will fall within 2 standard deviations of the mean. Standard deviation symbol is the sigma σ

So 2 standard deviations to the left of the mean is 2.5% of people who are "a lot less than normal" and 2 standard deviations to the right of the mean are 2.5% of people who are "a lot more than normal". These 2 bits are called "the tail".

You may see that the higher the standard deviation the wider the bell curve must be (to make sure 2.5% of all fall in the left and right tails).

You may also notice that this 2.5% is an arbitrary number. It defines 5% as being outside normal. Always 5%. Even if 99% were "normal" (as in say no disease) we would still have 2.5% above and 2.5% below.

So yes there are different testing machines but the units are standardised across countries. The range in your population simply represents the correct range to find 2.5% of people below, and 2.5% of people above. There is even a note

*"The following values apply to the so-called IFCC method at 37 ° C, based on the results of a major Nordic survey 5 and used by Danish laboratories"* There are plenty of Hep C patients who are (untreated) in your normal range, so it is a crude tool.

With ALT 10-70, the middle of the range is 40 so you are 10% below that and very close to the fat bit in the middle of the normal range.