The JW indicates the wasted amount. It goes on a separate line from the amount given.
On line one of your claim list the J code and the number of units to indicate the dose given. For example if 2 units = 10 mg and only 5 mg was given, put the J code on line one with 1 unit; the second line would be the J code with a JW modifier with 1 unit for the wasted amount.
No, that is not quite worded correctly and could be misconstrued or misinterpreted.
Regarding partial units administered. You always round up units as you can't bill fractions of units. Let's take Kenalog. Kenalog (J3301) comes in larger multi-use vials. In other words, you can pull multiple doses/administrations from the same vial. It is not a single-use vial or container. For example, a vial may contain 5 or 10 ml of Kenalog at a concentration of 40mg/ml or 10mg/ml. If one unit of J3301 is 10 mg, and you administer 15 mg, you round up to 2 units (20 mg). You do not use JW in this instance to disclose the rounded up dosage. JW is inappropriate to J3301 as Kenalog isn't distributed in single-use vials.
Here is a JW Modifier FAQ
https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/HospitalOutpatientPPS/Downloads/JW-Modifier-FAQs.pdf
Look at Question 8
Q8. Is the JW modifier applicable when the dose administered is less than the HCPCS billing unit?
A8. CMS does not use fractional billing units to pay for Part B drugs. Therefore, the JW modifier
should not be used when the actual dose of the drug administered is less than the HCPCS billing unit.
Let's make the response a little clearer...
JW is used for
single use vials when not all of the drug in the vial is administered to the patient.
For example, the single use drug is packaged in a 10 ml vial. You administer 5 ml of the drug, and have to discard the other 5 ml since it's single use. You bill for how ever many units you administer (paying attention to the HCPCS code and dosage per unit) and use JW on the amount you discard.
Here is a JW modifier fact sheet.
https://med.noridianmedicare.com/web/jddme/policies/dmd-articles/jw-modifier-use-010117