Plasma-Adapted First-Line Pembro In NSCLC

This research study is studying to see if a blood test, collected at different times during the treatment of metastatic non-small lung cancer, can be used to detect early response in patients being treated with pembrolizumab and use that information to determine whether patients should continue treatment with pembrolizumab or switch treatment to pembrolizumab in combination with chemotherapy. The names of the study drugs involved in this study are: * Pembrolizumab * Platinum doublet chemotherapy, which may include the following: * Carboplatin and pemetrexed * Carboplatin and paclitaxel The name of the blood test: \- InVision (Inivata, Ltd.)

Inclusion Criteria: \- Participants must have histologically or cytologically confirmed stage IV NSCLC (AJCC 8th edition). \- Participants must have evaluable disease on imaging per RECIST (measurable disease is not required). * No prior treatment with a systemic anti-cancer therapy of any kind for the treatment of stage IV NSCLC. Prior definitive chemoradiation for locally advanced disease, or prior adjuvant or neoadjuvant therapy for early stage disease is permitted if completed ≥6 months prior to initiating study treatment. * Age ≥18 years. * ECOG performance status 0-2 (see Appendix A) * Candidate for combination chemoimmunotherapy per physician assessment. * Participants must have normal organ and marrow function as defined below: \-- absolute neutrophil count ≥1000/mcL * platelets ≥100,000/mcL * total bilirubin \