ASA score
ASA stands for American Society of Anesthesiologists. In 1963 the ASA adopted a five category physical status classification system for assessing a patient before surgery. A sixth category was later added. These are:
- A normal healthy patient.
- A patient with mild systemic disease.
- A patient with severe systemic disease.
- A patient with severe systemic disease that is a constant threat to life.
- A moribund patient who is not expected to survive without the operation.
- A declared brain-dead patient whose organs are being removed for donor purposes.
If the surgery is an emergency, the physical status score is followed by E (for emergency) for example 3E. Category 5 is always an emergency so should not be written without "E". The category 6E probably does not exist. The original definition of emergency in 1940, when ASA classification was first designed, was "a surgical procedure which, in the surgeon's opinion, should be performed without delay". This gives an opportunity to the surgeon to manipulate the schedule of surgery for personal convenience. An emergency is now "defined as existing when delay in treatment of the patient would lead to a significant increase in the threat to life or body part. If it is correct then severe pain due to broken bones, ureteric stone or parturition is not emergency.
These definitions appear in each annual edition of the ASA Relative Value Guide. There is no additional information that can be helpful to further define these categories.

# Limitations and proposed modifications
Different authors give different versions of this ASA definition.  It is because this classification is vague and far from perfect. Many authors try to explain it on the basis of 'functional limitation' or 'anxiety' of patient which are not mentioned in the actual definition. Often different anesthesia providers assign different scores to the same patient. For example, heart attack (myocardial infarction), though grave, is a 'local' problem and is not a 'systemic' disease, so a patient with recent (or old) heart attack, in the absence of any other systemic disease, does not truly fit in any category of the ASA classification, yet having poor post-surgery survival rates. Other severe heart, liver, lung or kidney diseases, although they greatly affect physical status of patient and outcome of surgery, cannot be labelled as systemic disease (which means a generalized disorder of the whole body like hypertension or diabetes mellitus). Local diseases can also change physical status but not be mentioned in ASA classification.
This scoring system assumes that age of the patient has no relation to physical fitness, which is not true. Neonates and very old people, even in the absence of any systemic disease, tolerate anesthesia and surgery badly in comparison to young adults. Similarly this classification ignores patients with malignancy (cancer). This scoring system could not be improved to a more elaborated and scientific form, probably because it is often used for price reimbursement.
Some anesthetists now propose that like an 'E' modifier for emergency, a 'P' modifier for pregnancy should be added to the ASA score.

# Uses
While anesthesia providers use this scale to indicate the patient's overall physical health or "sickness" preoperatively, it is regarded by hospitals, law firms, accrediting boards and other health care groups as a scale to predict risk, and thus decide if a patient should have  or should have had  an operation.To predict operative risk, age and obesity of the patient, the nature and severity of the operative procedure, selection of anesthetic techniques, the competency of the surgical team (surgeon, anesthesia providers and assisting staff), duration of surgery or anesthesia, availability of equipment, medicine, blood, implants and especially the level of post-operative care etc. are often far more important than simple ASA score.

# History
In 1940-41, ASA asked a committee of three physicians (Meyer Saklad, M.D., Emery Rovenstine, M.D., and Ivan Taylor, M.D.) to study, examine, experiment and devise a system for the collection and tabulation of statistical data in anesthesia which could be applicable under any circumstances. This effort was the first by any medical specialty to stratify risk for its patients. While their mission was to determine predictors for operative risk, they quickly dismissed this task as being impossible to devise. They state:
"In attempting to standardize and define what has heretofore been considered 'Operative Risk', it was found that the term ... could not be used. It was felt that for the purposes of the anesthesia record and for any future evaluation of anesthetic agents or surgical procedures, it would be best to classify and grade the patient in relation to his physical status only."
The scale they proposed addressed the patient's preoperative state only, not the surgical procedure or other factors that could influence surgical outcome. They hoped anesthesiologists from all parts of the country would adopt their "common terminology," making statistical comparisons of morbidity and mortality possible by comparing outcomes to "the operative procedure and the patient's preoperative condition".
They described a six-point scale, ranging from a healthy patient (class 1) to one with an extreme systemic disorder that is an imminent threat to life (class4). The first four points of their scale roughly correspond to today's ASA classes 1-4, which were first published in 1963. The original authors included two classes that encompassed emergencies which otherwise would have been coded in either the first two classes (class 5) or the second two (class 6). By the time of the 1963 publication of the present classification, two modifications were made. First, previous classes 5 and 6 were removed and a new class 5  was added for moribund patients not expected to survive 24 hours, with or without surgery. Second, separate classes for emergencies were eliminated in lieu of the "E" modifier of the other classes.  The sixth class is now used for declared brain-dead organ donors. Saklad gave examples of each class of patient in an attempt to encourage uniformity. Unfortunately, the ASA did not later describe each category with examples of patients and thus actually increased confusion.