Blood test results – platelets, ALT, AST vary over time. If you have a look at this:

You will see the results of people taking Drug A, Drug B and Placebo (nothing).

Note that the values wander up and down. Note that the solid lines are the average values (for 150 people in each group) but that the vertical bars represent the RANGE of values seen. Even with the buffering of a large group smoothing it out the values wander.

Also note that some tests, from different labs report different “normal ranges” which have to do with the test being used and its standardisation. Although test from different labs are roughly comparable it’s almost certain that if I had blood taken at Lab A and 5 minutes later taken at Lab B the results would be slightly different. A number of people have been worried by this.

So if your ALT doubles, feel free to panic, but if it goes from 10 to 15 don’t worry yourself unnecessarily.

There’s a reason it’s called a “normal range” – a bit of variation day to day is completely normal.