There is no right or wrong answer to this. If you are currently taking a vitamin D supplement and want to check the impact of your supplementation routine, then you can take the vitamin D test straight away.
However, if you’re looking to discover your ‘baseline’ vitamin D levels (diet and lifestyle alone), then we would recommend a ‘wash-out’ period of 4-6 weeks, as this is around how long 25(OH)D (vitamin D) stays in the body.