Sure. As a non-expert, I assume most drops of my blood have enough stuff in them to detect. How much can vitamin levels or viral loads change between drops of blood? Certainly they seem to draw more than strictly needed.
It depends on what you are looking for. Yes, for say small molecules like vitamins a pinprick (less than a microliter) could theoretically be enough. But if you are looking for things like early infection or cancer, the signals you want may not actually be in such a small sample at all.