by Alexander C. Mueller
You might have a medical diagnosis you find embarrassing or just plain don’t want to talk about. If someone had your medical records, they would certainly find out… but if they had just some, any medical records, without reference to you or any other particular person, then your privacy is secure.
This silly fact highlights the difference between PII and PHI, and why PII is important. PHI, or personal health information, might be described as medical records that with enough data to tie them to people in the real world. This connecting data is then PII, or personally identifiable information, pretty much by definition. With your PII, a chart becomes your medical record and fits the definition of PHI, otherwise it is some medical record and this extra level of privacy motivates interest in deidentifying datasets.
PHI and PII overlap in usage a ton because PII is a key part of what makes PHI a privacy concern.