Here is one mention (click me for full reference):
United Healthcare Medicare Advantage uses the codes indicated in the CMS Place of Service (POS) Codes for Professional Claims Database to determine if laboratory services are reimbursable. The POS designation identifies the location where the laboratory service was provided, except in the case of an Independent or a Reference Laboratory.
An Independent or Reference Laboratory must show the place where the sample was taken (if drawn in an Independent Lab or a Reference Lab, POS 81 is reported; if drawn in a hospital inpatient setting, the appropriate inpatient POS is reported).
All entities billing for laboratory services should append identifying modifiers (e.g., 90), when appropriate, in accordance with correct coding. For example:
•If the physician bills for lab services performed in his/her office, the POS code for "Office" is reported.
•If the physician bills for a lab test furnished by another physician who maintains alab in his/her office, the code for "Other Place of Service” is reported.
•If the physician bills for a lab service furnished by an independent lab, the code for "Independent Laboratory" is reported.
•If an independent lab bills, the place where the sample was taken is reported. An independent laboratory taking a sample in its laboratory shows "81" as place of service.
•If an independent laboratory bills for a test on a sample drawn on an inpatient or outpatient of a hospital, it reports the code for the inpatient (POS code 21) or outpatient hospital (POS code 22), respectively.