The 91 modifier should only be used if the exact same testing is being performed on different specimens for medically necessary reasons. It should not be used to indicate multiple units. CPT code 82542 is for unspecified LC/MS testing not elsewhere specified and 83925 is for the Opiate confirmation so if you are billing either of these codes more than once a day I am assuming it is due to different drugs being tested which would make the 91 modifier inappropriate.
If different drugs being tested is the reason for the use of the 59 modifier and you report the 91 modifier on your claims you are indicating to the carrier that the test in question was the same test and that it was separately performed more than once to obtain comparison results.
If a carrier is indicating they will only pay for a 91 modifier the appropriate action would be to appeal with backup documentation showing why the use of the 91 would not be appropriate for your services. This may require taking the appeal to the highest level.
Laboratory billing for DOA are definitely under scrutiny and improper use of either codes or modifiers will result in unwanted audits and/or recoupments, which no one wants.
Bellow is from the CMS claims processing manual on the proper use of the 91 modifier:
C. Modifier ?-91?
Definition - The ?-91? modifier is used to indicate a repeat laboratory procedural service on the same day to obtain subsequent reportable test values. The physician may need to indicate that a lab procedure or service was distinct or separate from other lab services performed on the same day. This may indicate that a repeat clinical diagnostic laboratory test was distinct or separate from a lab panel or other lab services performed on the same day, and was performed to obtain subsequent reportable test values.Rationale - Multiple laboratory services provided to a patient on one day by the same provider may appear to be incorrectly coded, when in fact the services may have been performed as reported. Because these circumstances cannot be easily identified, a modifier ?-91? was established to permit claims of such a nature to bypass correct coding edits. The addition of this modifier to a laboratory procedure code indicates a repeat test or procedure on the same day.
Instruction - The additional or repeat laboratory procedure(s) or service(s) must be identified by adding the modifier ?-91?.
EXAMPLE 1:
When cytopathology codes are billed, the appropriate CPT code to bill is that which describes, to the highest level of specificity, what services were rendered. Accordingly, for a given specimen, only one code from a family of progressive codes (subsequent codes include services described in the previous CPT code, e.g., 88104-88107, 88160-88162) is to be billed. If multiple services on different specimens are billed, the ?-91? modifier should be used to indicate that different levels of service were provided for different specimens. This should be reflected in the cytopathologic reports.
Good Luck.