The ASPEN diagnostic criteria for refeeding syndrome help evaluate the severity of refeeding syndrome based on the decrease in serum levels of certain electrolytes—potassium, phosphorus, and magnesium. Here’s how the assessment works:

- You observe and measure the decrease in the serum levels of potassium, phosphorus, or magnesium post-feeding.
- You then choose from one of the predefined options representing the degree of decrease: 
  - A decrease of "10-20%" suggests mild refeeding syndrome.
  - A decrease of "20-30%" points towards moderate refeeding syndrome.
  - A decrease of "> 30% and/or presence of organ dysfunction" indicates severe refeeding syndrome.

The process involves matching your observation to these options to classify the condition:

- If the decrease in serum levels is between 10% to 20%, it meets the criteria for mild refeeding syndrome.
- If it falls between 20% to 30%, it is classified as moderate.
- If the decrease exceeds 30% or there’s subsequent organ dysfunction, it validates a severe refeeding syndrome diagnosis.

This systematic approach allows healthcare professionals to gauge the severity of refeeding syndrome efficiently, thereby facilitating timely intervention based on the critical electrolyte imbalance observed.

The calculator in question is the ASPEN Diagnostic Criteria for Refeeding Syndrome. ASPEN stands for the American Society for Parenteral and Enteral Nutrition. This scoring system is used specifically in patients who are malnourished or at risk of malnutrition, and who are about to undergo refeeding, either orally, enterally, or parenterally. 

The clinical utility of the ASPEN Diagnostic Criteria for Refeeding Syndrome is to identify patients at risk of developing refeeding syndrome, a potentially fatal condition characterized by severe electrolyte and fluid shifts in malnourished patients as they receive artificial refeeding. Early identification of these patients allows for careful monitoring and adjustment of nutritional therapy to prevent the onset of the syndrome.

There are no specific exclusion criteria for the use of the ASPEN Diagnostic Criteria for Refeeding Syndrome. However, it is not applicable to patients who are not malnourished or at risk of malnutrition, or those not undergoing refeeding.