On-Q-ity raised $21 million in a Series A round to develop blood-based diagnostics used to monitor treatment of cancer patients.

The Waltham, Mass.-based firm raised the cash from venture backers including Bessemer Venture Partners, Physic Ventures and Northgate Capital, led by MDV-Mohr Davidow — which drove the creation of the company from a pair of venture-backed firms that couldn’t raise their Series B rounds last summer, according to the PE Hub blog.

CEO Mara Aspinall, the former president of Genzyme Genetics, was brought on by Cellective early this year after its Series B round foundered.

On-Q-Ity’s cancer cell detection and testing platform is based on the combined technologies of Cellective Diagnostics and The DNA Repair Co., both Mohr Davidow portfolio companies that struggled to raise capital last year. The system uses tumor samples and its proprietary circulating tumor cell technology to detect and monitor treatment-resistant cells for molecular changes. The company claims the technology can deliver real-time data about the course of treatment for cancer patients.

“This allows us to do a lot less invasive testing with a very simple blood draw that may well be done on a regular basis for other reasons,” Aspinall told PE Hub. “We can educate patients to be much more familiar with the intricacies of their care, and there’s the ability to guide a treatment regimen using all the available drugs. Doctors and patients can choose the best one.”

On-Q-ity, which is in large-scale clinical trials and expects to have tests on the market by 2011, will focus on breast and thoracic cancers first, then prostate and other solid tumor cancers, according to the blog.