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.
First, Excel has a row limit of 65,536. If the combined amount of data exceeds this amount, you will not be able to store it in one Excel file. Second, CSV and XLS are fundamentally different. CSV is a text-only file that you can open with any text-editor. XLS files are binary representations of all kinds of data, text being only a small part, that are generally only supported by Excel and other spreadsheet applications.