Published: 14 Jan 2024

These include:
Certain foods and drinks
Medicines
Stress
Vigorous exercise
Variations in lab procedures
Having an illness
If you have any questions about your lab tests or what your results mean, talk to your health care provider.
A false positive result means your test shows you have a disease or condition, but you don't actually have it.
A false negative result means your test shows you don't have a disease or condition, but you actually do.
These incorrect results don't happen often, but they are more likely to happen with certain of types tests, or if testing was not done right. Even though false negatives and positives are uncommon, your provider may need to do multiple tests to make sure your diagnosis is correct.
What factors can affect my results?
There are many factors that can affect the accuracy of your test results.

This is why the term 'reference range' is preferred over 'normal range'.
When you examine test results from different populations, you quickly discover that what is 'normal' for one group is not necessarily normal for another group. Indeed for tests such as cholesterol the idea of a normal range has been replaced to a large extent by use of target values, achieved either by lifestyle changes or active treatment.
Whether or not your test result is within the laboratory reference range, the result must be considered within the context of your personal circumstances, and with the benefit of your doctor's knowledge of your past medical history, current medication and the results of any other investigations.The reference range is then derived mathematically by taking the average value for the group and allowing for natural variation around that value (plus or minus 2 standard deviations from the average). The first step in determining a reference range is to define the population to which the range will apply. A large number of individuals from a group who are thought to represent a 'normal' population, will be tested for a particular laboratory test. In this way, ranges quoted by labs will represent the values found in 95% of individuals in the chosen 'reference' group. In other words, even in a 'normal' population, a test result will lie outside the reference range in 5% of cases (1 in 20).