Namely, the CS modifier needs to be applied on each applicable claim line. This is needed to identify the procedure as subject to the cost-sharing waiver for COVID-19 testing-related services. Providers should not charge Medicare patients any co-insurance and/or deductible amounts for services described in Section 1135 of the Social Security Act by HHS.
At the same time, this modifier does not need to be applied to tests performed in a laboratory.
Source: ngsmedicare.com