Calculating the critical stock level (or ‘time to order’)

It is important to be aware of your vaccine stock, and to place your next order at the right time. The critical stock level (or ‘time to order’ level) is the number of vaccine doses in stock at the time when it is absolutely necessary to place a new order. ‘Time to order’ calculations take into account the level of vaccine consumption while waiting for the new supply. This precaution is necessary to prevent the vaccine stock from dropping below the minimum stock level before the new order can be collected. The delivery time is the time interval between the day the vaccines are ordered and the day that you collect them.

The critical stock level can be calculated using the following equation:

• s_critical = q_delivery + s_mini , where:

• s_critical is the critical stock level
• q_delivery is the number of doses needed during the delivery time, and
• s_mini is the minimum stock level.

• q_delivery = (q_period x t_delivery) ÷ n , where:

The number of doses required during the delivery time (q_delivery) can be calculated using the following equation:

• t_delivery is the number of days between placing the order and collecting new vaccines
• n is the number of days in the supply period (i.e. the period that the health facility maintains its vaccine stocks at, or above, the minimum level).

Now attempt the activity below to calculate the critical stock level for an imaginary health facility. You will need paper and a pen or pencil to help you make the calculations.

The calculation of the critical stock (or ‘time to order’) level can be done using the equations given above. First, calculate the number of doses you will need during the delivery time (q_delivery) by using the equation:

• q_delivery = (q_period x t_delivery) ÷ n

In this health facility, q_period is 80 doses of OPV, t_delivery is 2 days, and the supply period is 10 working days (2 weeks) so n is 10.

• q_delivery = (80 x 2) ÷ 10 = 16 doses

The minimum stock level (s_mini) is 25% of 80 doses, which is 20 doses. The critical stock level (s_critical) is calculated using the equation:

• s_critical = q_delivery + s_mini

In this health facility, q_delivery is 16 doses and s_mini is 20 doses, so:

• s_critical = (16 + 20) = 36 doses.

So the staff at this health facility should place an order for OPV when the critical stock level is reached at 36 doses.