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.
In .csv only the values from within your worksheet are stored. No formulas, no styling, just values. In .xlsx your data is stored in the native Excel format, which means that also your formulas and cell and value styles are preserved when you open it again in Excel.