The amount of time required for the removal of a tattoo and the success of the removal varies with each individual and their immune system function. Factors influencing this include: skin type, location, color, amount of ink, scarring or tissue change, layers of ink, immune system function and circulation. Factors under the individuals control are more time between treatments, nutrition, stress, sleep, exercise and fluid levels. In the past health care providers would simply guess on the number of treatments a patient needed which was rather frustrating to patients. A predictive scale, the "Kirby-Desai Scale", was developed by Dr. Will Kirby and Dr. Alpesh Desai, dermatologists with specialization in tattoo removal techniques, to assess the potential success and number of treatments necessary for laser tattoo removal, provided the medical practitioner is using a Q-switched Nd:YAG (neodymium-doped yttrium aluminium garnet) laser incorporating selective photothermolysis with six weeks between treatments. 
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