Q. Is sterilization safe for my puppy Shih Tzu? She's had her blood tests done
A. Yes, a spay is a somewhat routine procedure. If her bloodwork is normal, then anesthesia should be fine, but make sure you take her to a reliable vet that has high quality equipment to monitor anesthesia. I would recommend taking her to a practice that has a certified veterinary technician performing the anesthesia.
Of course, there is always some risk with any procedure that involves anesthesia, but if bloodwork is normal and you find a good vet you can trust, then you should have no issues.