All scales become inaccurate after time and must be calibrated, or made more accurate. This process is different with each scale, and some home-model scales do not even have methods of calibration. However, since it is crucial that doctors' scales are accurately calibrated, doctors' scales always include a knob or other method of calibration.
Not all scales calibrate in exactly the same manner, although there are general principles that are the same in all doctors' scales. You should calibrate your scale as often as possible to preserve its accuracy.
Set an object with a set weight on the scale. This object can be a 12-pound bowling ball, a barbell, or any other object that you can use as a standard.
Take note of the weight that the scale gives. This is what the scale is reading when you weigh objects.
Subtract the amount of the known weight from this number. For example, if your scale says that your 12-pound bowling ball is 14 pounds, subtract 12 from 14. This example results in 2 pounds, which is the amount that the scale is off.
Turn the calibration knob (or other method of calibration that your particular doctors' scale uses) until the scale reads exactly what the weight of the known object is. Much of this step is trial and error.
Repeat this process at regular intervals to ensure the highest degree of accuracy in your scale.