Presumptive tests are just that. Presumptive. You would in Scotland at least, seize the potential drug material, arrest and charge the person with a Section 5(2) of the Misuse of Drugs Act and inform them that it will be presumptively tested and that -should- it give a positive result, that a report would be sent to the Procurator Fiscal. You then take it back to the station and presumptively test it in the office, and if you're not happy or it's not a drug type that can be tested, you send it off to forensics.
You cannot arrest someone and convey them to a custody suite to spend the night in the cells prior to going in front of a Sheriff in the morning unless it's dealer quantities, which are substantial to say the least. Only time I've ever had that was when some poor lad got on the train I and my colleague had hopped on coming back from Court. Poor sod didn't realise we were there until the doors shut, and then the smell hit us. Whoops!