When did she say her period was due ??

In my opinion, the only reason the GP may have said the blood test may be inaccurate, is that it is too soon for a HCG level to show, even via a BT. IE: If it was a couple of days before her period. She may have been "telling white lies" about the urine tests, and the GP is sending her for a scan to double check there is a pregnancy.

An internal ultrasound, from my experience, will usually show a "gestational sac" from around 5.5 weeks.