Warfarin concentrations may be affected when co-administered with Kaletra due to CYP2C9 induction. It is recommended that INR (international normalised ratio) be monitored.
Kaletra Summary of Product Characteristics, AbbVie Ltd, January 2021.

Warfarin concentrations may be affected when co-administered with Kaletra. Initial frequent monitoring of the INR during Kaletra and warfarin co-administration is recommended.
Kaletra Prescribing Information, AbbVie Inc, October 2020.

The effect of lopinavir/ritonavir (400/100 mg twice daily) on the metabolism of S-warfarin (10 mg single doses in combination with omeprazole and vitamin K) was studied in 7 healthy volunteers. Coadministration resulted in a decrease in warfarin AUC from 14.69±6.93 to 11.36±4.71 µg/ml.h Increased doses of drugs metabolised primarily by CYP2C9 may possibly be necessary for optimal management when given concomitantly with lopinavir/ritonavir.
Lopinavir/ritonavir induces CYP2C9 and 2C19 activity as measured by warfarin and omeprazole biomarkers in healthy human volunteers. Yeh R, Gaver V, Park J, et al. 5th International Workshop on Clinical Pharmacology of HIV Therapy, Rome, April 2004, abstract 4.1.

A case has been described of a patient on warfarin and lopinavir/ritonavir. The patient was formerly on ZDV, 3TC and lopinavir/ritonavir, but this was stopped for surgery (mitral valve replacement). Following surgery, a daily dose of warfarin between 3.75 and 5 mg was required to maintain an optimal INR after. When the antiretrovirals were reintroduced, a progressive increase in warfarin dose to 10 mg daily was required. The authors suggest that ritonavir is inducing CYP2C9.
Drug interactions between warfarin and efavirenz or lopinavir/ritonavir in clinical treatment. Bonora S, Lanzafame M, D’Avolio, et al. Clin Infect Dis, 2008, 46: 146-147.

There is a case report of an HIV+ patient on warfarin whose INR decreased substantially following the initiation of lopinavir/ritonavir. Prior to commencing lopinavir/ritonavir his INR was stable between 2 and 3 whilst on other drugs including fluconazole, but in the presence of lopinavir/ritonavir it dropped to between 1.0 and 1.3.Dose titration to 11 mg/day did not achieve a therapeutic INR; however when the fluconazole was discontinued, the INR increased to 2.6. The patient eventually attained a stable INR between 2 and 3 with a warfarin dose of about 13 mg/day.
Interaction between lopinavir/ritonavir and warfarin. Hughes CA, Freitas A, Miedzinski LJ. Can Med Assoc J, 2007, 177(4): 357-359.