Loss of appetite doesn’t always mean that a person will be diagnosed with an eating disorder. According to your data about height, sex and age your ideal weight is 49kg. Loss of more than 10% of ideal weight is considered "underweight". In your case you’ve lost approximately 12% of your ideal body weight. Measuring the perimeter of the arm and thickness of the skin there help nutritionists determine mal/nutrition.

There are several laboratory analyses that can help diagnose eating disorders as well: creatinin in urine for 24 hours, transferin level in blood, blood albumin, number of lymphocytes, skin sensitivity tests, complement(C3)-level etc. Rather than taking a test online, you might consider going to your doctor's office for medical examinations. You can discuss levels of confidentiality with the office staff before your visit.