Table IV. Reasons for false positives and false negatives in the best NLP system.

False Positives
Category	Example
Suspected PAD	“…he does have palpable dorsalis pedis pulses of the feet bilaterally with a difficult to palpate right posterior tibial pulse. Noninvasive arterial studies of the lower extremities will be performed to evaluate the severity and extent of peripheral arterial disease.”
Ambiguity and complexity of natural language	“…She lives with her husband who is status post renal transplant in 1999. He struggles with diabetes and arteriosclerosis in both lower extremities.”
False Negatives
Absence of location and/or diagnostic keywords	“…she clearly has peripheral arterial disease, but the left is worse than the right…”
ABI results not reported in clinical notes due to recently developed acute health problem	“Diminished pedal pulses: We will obtain noninvasive vascular studies of the lower extremities (September 12, XXXX)”. ABI testing completed and reported on September 18, XXXX as mildly abnormal but not mentioned in the subsequent clinical notes. On the same day, the patient had new symptoms, was diagnosed with new atrial fibrillation, and underwent comprehensive cardiovascular evaluation for assessment of this acute condition.
Typographic errors	Instead of the correct term “ABI” clinical note snippet contains a typographic error “ADI”: “…patient also has an appointment with the cardiologist today for further evaluation of his abnormal ADI.”