I always had the best, earliest and most consistent results / luck with the cheaper, store brand tests (CVS, in my case).
An HCG of 51 is definitely positive. Let's assume, for a split second that something is amiss- it takes days and days for the HCG to drop to the point of no longer registering on a home pg test. It doesn't drop like an on/off switch.
So it is the test, in my opinion. I hope that helps.