The time between symptom recognition and a definitive diagnosis have been described in many terms including ‘Time-to-Diagnosis’ (TtD), delay to diagnosis, pre-diagnostic interval and duration of symptoms in the literature  and we will use TtD across this paper. The concept of the term “delay” assumes that there is an ideal period of time to arrive to a diagnosis in a patient with cancer
Pack and Gallo in their seminal work published in 1938  were the first one to define what they call “reasonable” delay on the part of the patient “when the time elapsing between the onset or discovery of symptoms and the first visit to a physician is under three months”. Since the word “delay” may give only importance to the element “time”, they use the term “criticism” in the evaluation of other physician´s responsibility. The words written almost eighty years ago are still valid “it will be clear that there is still room for improvement in the service rendered by the medical profession to patients suffering from cancer. Many investigators stress the fact that the responsibility lies with the physician. The physician first seeing the patient largely determines the outcome according to Rector”. Recent articles have shown in practical terms that a three months period of time could be a useful “point in time” to divide a population of colorectal cancer patients in two equal halves [8-9]. However, there are large variations in the definition of delay in the literature and with the incorporation of new theoretical models a simple definition can be not only difficult but inaccurate. The variable selected as “delay” should have the possibility to be stratified in different levels of risk, something that we can do with “time” and “number of visits”.
Time measured in days, can be analyzed as a continuous variable and summarised using mean, median and other quintile (e.g. 75th, 90th, 95th) centiles, or can be parameterised using distinct interval categories.
A common categorization is to organize data in four “quartiles” with the 25th and 75th quartiles to represent short and long delays respectively, but only one type of cancer must be included inside each category, because mixing different tumours with different biological behaviour can be misleading [12-15].
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