As mentioned on many other GymProfessor.com web pages, prior to undertaking fitness tests with your client, you should be fully qualified, have a satisfactorily completed PAR-Q with informed consent, & appropriate insurance.
Your client should also adhere to the following pre-test precautions:
Wear comfortable clothing.
Have a good nights sleep the day before.
Avoid drinking alcohol the day before and on the day of assessments.
Avoid heavy meals less than 2 hours before assessments.
Avoid smoking at least 2 hours prior to the assessments.
Avoid caffeine drinks less than 2 hours before assessments.
Avoid strenuous physical activity on the day of assessments.
If unwell, reschedule assessments.
Since first writing these pre-test precautions many years ago, I've found this brief article cross-referenced time-and-time-again. If you have found it helpful too, please share it with your friends (and quote the GymProfessor.com as the source please).