Laboratory reference ranges can be set by the manufacturer of the analyser (the equipment that tests your sample) or the laboratory that tests your sample. The “normal" range for any given population (the people they have tested) is usually the range where 95% of that population will fall within. This means that 2.5% of that population will be below the normal range, and 2.5% would fall above the normal range.
At the edges of the normal range, there are areas of overlap where healthy individuals will have results outside of the normal range and unwell individuals will have results inside the normal range. This is why our doctors will normally advise you to repeat a borderline result in a few weeks or months to see whether that result is normal for you, or to see whether it has improved or got worse. Lots of things can affect your blood test results, including what you recently ate or drank, whether you have an allergy or infection, and whether you have recently exercised. This is why it always important to repeat any abnormal test result.