In addition, we are seeking opportunities to improve upon Cologuard’s performance characteristics. For example, we are evaluating whether new biomarkers and improved laboratory processes would increase specificity while maintaining sensitivity. If we could increase the specificity of Cologuard, we believe that would enhance its adoption as a front-line screening test. We are also evaluating ways that we might make Cologuard even easier for patients to use and potential opportunities to lower the cost of goods sold for Cologuard.
The timing of any such enhancements to Cologuard is unknown and would be subject to FDA approval.
Advance Liquid Biopsy
We are also focusing our research and development efforts on building a pipeline of potential future products and services with a focus on blood or other fluid-based (“liquid biopsy”) tests. We will continue to advance liquid biopsy through biomarker discovery and validation in tissue, blood, or other fluids. We have identified proprietary biomarkers for several cancers, including liver cancer and pancreatic cancer. Through our collaboration with Mayo Foundation for Medical Education and Research, we have successfully performed validation studies on tissue samples for thirteen cancers and on blood or other fluid samples for nine cancers.
Results of Operations
We have generated significant losses since inception and, as of September 30, 2019, we had an accumulated deficit of approximately $1.2 billion. We expect to continue to incur losses for the near future, and it is possible we may never achieve profitability.
Revenue. Our consolidated revenue is primarily generated by performing screening services using our Cologuard test. For the three months ended September 30, 2019 and 2018, we completed approximately 456,000 and 241,000 Cologuard tests, respectively, and generated revenue of $218.8 million and $118.3 million, respectively. For the nine months ended September 30, 2019 and 2018, we completed approximately 1.2 million and 0.6 million Cologuard tests, respectively, and generated revenue of $580.7 million and $311.5 million, respectively. The increase in revenue was primarily due to an increase in completed Cologuard tests during the current period primarily due to increased selling and marketing efforts.
Our cost structure. Our selling, general and administrative expenses consist primarily of non-research personnel salaries, office expenses, professional fees, sales and marketing expenses incurred in support of our commercialization efforts and non-cash stock-based compensation.
Cost of sales includes costs related to inventory production and usage, shipment of test collection kits, royalties and the cost of services to process tests and provide results to physicians. We incur expenses for tests in the period in which the activities occur, therefore, gross margin as a percentage of revenue may vary due to costs being incurred in one period that relate to revenues recognized in a later period.
We expect that gross margin for our services will continue to fluctuate and be affected by Cologuard test volume, our operating efficiencies, patient compliance rates, payer mix, the levels of reimbursement, and payment patterns of payers and patients.
Cost of sales. Cost of sales increased to $52.8 million for the three months ended September 30, 2019 compared to $30.0 million for the three months ended September 30, 2018. Cost of sales increased to $147.6 million for the nine months ended September 30, 2019 compared to $79.8 million for the nine months ended September 30, 2018. The increase in cost of sales is primarily due to the increase in completed Cologuard tests. The Company completed approximately 456,000 and 241,000 Cologuard tests for the three months ended September 30, 2019 and 2018, respectively. The Company completed approximately 1.2 million and 0.6 million Cologuard tests for the nine months ended September 30, 2019 and 2018, respectively.