Kits and methods providing measurement of tumor burden in a patient derived xenograft (PDX) mouse are described. Exemplary embodiments contemplate taking a sample, typically a blood sample, from a PDX mouse and using a real-time polymerase chain reaction (PCR) system to quantitate both human patient circulating tumor DNA (ctDNA) and mouse DNA. In preferred embodiments, both PCR amplifications are done simultaneously in a multiplex, and a highly polymorphic human DNA target sequence is amplified for high sensitivity, allowing for small volume samples, typically 50-100 μL, of mouse blood. Serial evaluations are possible because the mouse can survive withdrawal of these small volumes of blood. A related method allows for quantitation of ctDNA in the presence of human immune cells added to a “humanized” mouse. These relatively quick and easy methods of determining tumor burden in PDX mice can have predictive value for the efficacy of cancer treatments in human patients.