Table 3.

Cross tabulation of the index test (ICD-9-CM code 172.x) results by the results of the reference standard (medical chart)

Operative unit	True positives	False positives	True Negatives	False Negatives
Unit 1 (Umbria)	99	13	92	0
Unit 2 (ASL Napoli 3 Sud)	89	27	93	1
Unit 3 (Friuli Venezia Giulia)	107	23	92	2

ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification.