The duration of neutropenia was the interval between when the absolute neutrophil count (ANC) drops below 500 per µL and when it recovers above that level.
Parameters:
(1) number of days below 500 neutrophils per µL
(2) absolute neutrophil count each day during this period
If the neutrophil count stayed at 500 per µL for N days, then the area would be:
area at 500 per µL =
= ((number of days) -1) * 500
area under the curve for the absolute neutrophil count =
= (0.5 * ((absolute neutrophil count day 1) + (absolute neutrophil count day 2))) + (0.5 * ((absolute neutrophil count day 2) + (absolute neutrophil count day 3))) +
…(0.5 * ((absolute neutrophil count day N-1) + (absolute neutrophil count day N)))
where:
• In the example given on page 3850 the author uses (N-1) to define the time interval. But the level on day 1 is already below 500, arguing for using a day 0.
• The assumption is that the ANC is >= 500 per µL on day N+1.
D index = area above the AUC curve =
= (area if ANC 500 per µL for N days) - (area under the curve for period of neutropenia)
The units for the areas are days•neutrophils/µL.
Interpretation:
• The worse the neutropenia - both in magnitude and duration - the greater the D-index.
• The greater the D-index the greater the risk for an opportunistic infection.
