CAC test is an acronym for Circulating Abnormal Cells Detection Technique for Lung Nodules. The CAC test is a technique for detecting abnormal cells in the blood circulation of lung nodules and for finding circulating chromosomal abnormal cells, and is intended primarily for early screening for lung cancer. Chromosomal abnormalities in human cells usually occur in the pre-cancerous stage of lung cancer, and chromosomal abnormalities can be a target for assessing the risk of malignancy in lung nodules. For this reason, the CAC test is often used in combination with fluorescence-labeled in situ hybridization to qualitatively detect chromosomal abnormalities in single nucleated cells in human peripheral blood to assist in the diagnosis of benign or malignant lung nodules. This testing technique is relatively safe and non-invasive, the amount of blood collected is small, usually only 10 ml of peripheral blood is needed; at the same time, the target point is clear; the accuracy is relatively high, clinical data show that generally ≤ 10 mm lung cancer detection accuracy is relatively high; compared with the traditional tumor markers, it has the characteristics of high sensitivity.