CDK4/6 inhibitors have led to an improvement of both progression free survival (PFS) and overall survival (OS) in patients with advanced estrogen positive (ER+)/HER2- breast cancer when applied in the first or second line of treatment. Despite the advantages of CDK4/6 inhibitors, these medications can lead to adverse effects. One of the adverse events observed across all types of CDK4/6 inhibitors is an elevation in creatinine levels. An elevation in plasma creatinine during treatment with abemaciclib is not always indicative of a reduction in renal function; it can also be attributed to the inhibition of active tubular secretion of creatinine. This phenomenon is known as pseudo acute kidney injury (pseudo-AKI).

The incidence of pseudo-AKI in patients using CDK4/6 inhibitors is currently unknown. A method to distinguish pseudo-AKI from AKI is measuring the level of an alternative filtration marker in blood, for example cystatin C. Cystatin C is also filtered at the glomerulus but not secreted intro the renal tubulus or reabsorbed into the bloodstream. Also, there is no affection by muscle mass or diet. In this study the investigators will explore the incidence of both AKI and pseudo-AKI in patients who are treated with CDK4/6 inhibitor treatment by assessing both creatinine and cystatin C in plasma.

Inclusion Criteria:

• Adult patients who are receiving CDK4/6 inhibitors for metastatic breast cancer
• Patients must have used the CDK4/6 inhibitor for a minimum duration of one month
• Patients must have undergone at least one measurement of creatinine after the initiation of CDK4/6 inhibitor treatment

Exclusion Criteria:

- Patients who are undergoing dialysis therapy for kidney failure.

Adult patients with metastatic breast cancer that are treated with a CDK4/6 inhibitor (i.e. palbociclib, ribociclib, abemaciclib).