Strange like so many things in the case of missing Maddie. Had they been so concerned they could have had analysis done at any time independent of the Portuguese police. Maybe they did have and it came up negative? Another DEAD end!!
It was only a question that they asked in view of the fact that 2-year-old twins didn't wake up despite the amount of noise.
A normal question to think of as a possibility, IMO.
Reality may be that they were simply exhausted after nearly a week of fun-filled activities.
Hacks with nothing else to do were all over the case - they even managed to fabricate that Madeleine's biological father was an anonymous sperm donor to a Birmingham lab. If the McCanns had had the twins tested privately early on... wouldn't that have also appeared in the media? The money flying about back then for scoops appears to have been extremely tempting.
If they had had the twins tested early on and it came up negative... how would that fit in with a non-abduction scenario? Why do so in the first place, having asked the police to check whether a sedative could have been used?