Creatinine in the blood increases with muscle mass, so men often have higher creatinine levels than women, the Mayo Clinic explains. A high serum creatinine level means your kidneys aren’t working well. So if your number is high, your doctor will order a GFR to see how much kidney function you have left. GFR may also be estimated from your blood level of creatinine. If your GFR falls below 30 you will need to see a kidney disease specialist (a nephrologist).