X in front of LOINC code

Hi, I am mapping the incoming external codes in EPIC happy together module. Does anyone know what “X” indicates in front of the LOINC code?

e.g. X11011-4 for HCV, Quant RNA

Thank you for your help in advance.

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Hi Gina,
You may want to inquire from the producing source. In the “old days” if a local site had an assay that wasn’t currently qualified for an existing LOINC, an X in front of a sequence number would populate the field, but be recognized as a non-LOINC. I haven’t seen it used in front of an actual LOINC code. I personally would want to rule out that an accidental concatenation occurred.

At the upcoming LOINC International Conference, there is a proposed cross-cutting panel discussion for new assignments of LOINC codes that may be a subsequent resolution to the older “x-codes” need of covering the temporary gap of a true LOINC.

Stay tuned!!


Hi Gina,

I will second Pam’s response. I saw this happen over 10 years ago when laboratory results were mapped for transmission from an outside reference lab, through an HIE, and into the healthcare system’s LIS. In that particular implementation, several different individual alpha characters were used over time. (e.g ‘x’, ‘N’, ‘Y’, etc.) as the prefix character to an otherwise valid LOINC number.

Depending on where you sit in the data stream, you may still see some of this alpha prefixed content.


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Echo what Pam & John said about the ‘producing source’ and the ‘particular implementation’ as the saying goes, “if you’ve seen one ehr instance, you’ve seen one ehr instance”.

On that note, I’ll add that in my previous implementations, ‘ex’ (or perhaps ‘x’) in this case was added for the reason John mentioned: it came from an outside reference lab.