Starting testing on/near Ovulation day?

Me and my girlfriend are wondering if it's around her ovulation today, so we bought the Clearblue Advanced Digital Ovulation Test, and took it for the first time and had a "low" reading. If it was during her ovulation, would it read as "high" or "peak" fertility for the first ever reading? Or does it only work by comparing changes in the hormone levels rather than just levels of the hormones. Thanks!