One touch ultra-mini reading fluctuation. 590+

My mother has been highly diabetic all her life (read 200+). Today her post breakfast readings ranged from 592, "HIGH", 520, 490. same blood sample showed readings 592 and 520. is that even possible to get those fluctuations? I tried it on myself and it showed 88. I immediately gave her an insulin shot and contacted her doctor. he asked her to admit take another reading and admit.