Introduction: Patients with ADTKD- MUC1 have one allele producing normal mucin-1 (MUC1) and one allele producing mutant MUC1, which remains intracellular. We hypothesized that ADTKD- MUC1 patients, who have only 1 secretory-competent wild-type MUC1 allele, should exhibit decreased plasma mucin-1 (MUC1) levels. To test this hypothesis, we repurposed the serum CA15-3 assay used to measure MUC1 in breast cancer to measure plasma MUC1 levels in ADTKD- MUC1. Methods: This cross-sectional study analyzed CA15-3 levels in a reference population of 6,850 individuals, in 85 individuals with ADTKD- MUC1, and in a control population including 135 individuals with ADTKD- UMOD and 114 healthy individuals. Results: Plasma CA15-3 levels (mean ± standard deviation) were 8.6 ± 4.3 U/mL in individuals with ADTKD- MUC1 and 14.6 ± 5.6 U/mL in controls ( p < 0.001). While there was a significant difference in mean CA15-3 levels, there was substantial overlap between the 2 groups. Plasma CA15-3 levels were <5 U/mL in 22% of ADTKD- MUC1 patients, in 0/249 controls, and in 1% of the reference population. Plasma CA15-3 levels were >20 U/mL in 1/85 ADTKD- MUC1 patients, in 18% of control individuals, and in 25% of the reference population. Segregation of plasma CA15-3 levels by the rs4072037 genotype did not significantly improve differentiation between affected and unaffected individuals. CA15-3 levels were minimally affected by gender and estimated glomerular filtration rate. Discussion/Conclusions: Plasma CA15-3 levels in ADTKD- MUC1 patients are approximately 40% lower than levels in healthy individuals, though there is significant overlap between groups. Further investigations need to be performed to see if plasma CA15-3 levels would be useful in diagnosis, prognosis, or assessing response to new therapies in this disorder.