ISI stands for Issueable Stock Index and is effectively an indicator for how many days stock you are holding in your inventory.
It is calculated as follows:
Over a six month period, ARCBS sum the total of a product eg red cells have distributed to you. Divide this by the number of working days in six months based on a six day week (108 days). This provides the average daily issue or ADI.
Then divide how much stock you have by the ADI and this provides the ISI.
Example:
Usage is 600 O positive red cells over six months
ADI = 600 / 108 = 5.55 units per day
If you have 20 bags in stock then the ISI is
20 / 5.55 = 3.6 days of stock
You should set your ideal stock level and ISI level in conjunction with your ARCBS Transfusion Medicine team.