In 2002, the National Kidney Foundation published treatment guidelines that identified five stages of Chronic Kidney Disease (CKD) based on declining GFR measurements. The guidelines recommend different actions based on the stage of kidney disease.
CSV (Comma Separated Value) was a file format devised to store tabular data. It uses commas to separate each entry in a row and the newline symbol to go to the next row. Excel is a more recent spreadsheet application from Microsoft and it saves the data with its own file format.
A .csv file is actually simple in structure; as the name suggests, it consists of data separated by commas. Because of this, it's a good idea to not include any commas in files intended to be exported to .csv, because this will mess with the file structure.