The three factors that can influence laboratory diagnostics include:
What you drink or eat before the test
The medicine you take before the test
Your sex or age
What you eat or drink can influence laboratory diagnostics. even the time you eat can affect it too. For instance, when checking for blood sugar level, it might be done while fasting or while still eating. fasting blood sugar level is mostly done in the morning, say 6 am because it is assumed that the patient has stopped eating by say 10 am. while random blood sugar is tested for at any time when the patient has eaten. The blood sugar level for the random test is expected to be higher than that of fasting.
the medicine you take before the test also matters. for instance, if a patient takes anti-malarial medicine before testing for material, the results cannot be reliable because it cannot reflect the true state of the patient. the result is not potent or viable.
the age and sex of the patient can influence the diagnostics. for in Packed Cell Volume, the percentage for the female is usually lower than that of the male.