into the appropriate workflow. This number is a subset of the number of tests that we process, which includes tests distributed through our Constellation licensees. The number of tests that we process is a key metric as it tracks overall volume growth, particularly as our laboratory partners may transition from sending samples to our laboratory to our cloud-based distribution model, as a result of which our tests accessioned would decrease but our tests processed would remain unchanged.
During the year ended December 31, 2021, we processed approximately 1,570,000 tests, comprised of approximately 1,513,400 tests accessioned in our laboratories, compared to December 31, 2020, in which we processed approximately 1,026,500 tests, comprised of approximately 974,400 tests accessioned in our laboratories, and approximately 804,300 tests processed during the year ended December 31, 2019, comprised of approximately 753,800 tests accessioned in our laboratories. This increase in volume represents continuous commercial growth of Panorama and HCS, both as tests performed in our laboratories as well as through our Constellation software platform.
The percent of our revenues attributable to our U.S. direct sales force were 89%, 87% and 80% for the years ended December 31, 2021, 2020, 2019, respectively. The percent of our revenues attributable to U.S. laboratory partners for the year ended December 31, 2021 was 5%, which was down from 7% and 6%, when compared to the years ended December 31, 2020 and 2019. The percent of our revenues attributable to international laboratory partners and other international sales for the years ended December 31, 2021 was 6%, consistent with 6% and down from 14% for the years ended December 31, 2020 and December 31, 2019, respectively.
For the year ended December 31, 2021, total revenues were $625.5 million, compared to $391.0 million and $302.3 million in the years ended December 31, 2020 and 2019, respectively. Product revenues generated from our testing accounted for $567.1 million or 91% of total revenues for the year ended December 31, 2021, compared to $367.2 million or 94% of total revenues for the year ended December 31, 2020 and $269.9 million or 89% of total revenues for the year ended December 31, 2019. For the years ended December 31, 2021, 2020, and 2019, there were no customers exceeding 10% of the total revenues on an individual basis. Revenues from customers outside the United States were $34.6 million, representing 6% of total revenues for the year ended December 31, 2021. For the years ended December 31, 2020 and 2019, revenues from customers outside the United States were $25.3 million and $41.5 million, representing approximately 6% and 14%, respectively, of total revenues.
Our net losses for the years ended December 31, 2021, 2020, and 2019, were $471.7 million, $229.7 million, and $124.8 million, respectively. This included non-cash stock compensation expense of $115.2 million, $50.2 million, and $28.6 million for the years ended December 31, 2021, 2020, and 2019, respectively. As of December 31, 2021, we had an accumulated deficit of $1.4 billion.
COVID-19 Impact
The COVID-19 pandemic has continued to present a global public health and economic challenge that has affected our business operations and the U.S. and other major economies and financial markets. We have modified our business practices in response to the spread of COVID-19 (including temporary closures of our offices, implementing remote work policies and practices, vaccination requirements, travel restrictions, and other measures as we have deemed necessary or appropriate from time to time), and incur additional operating costs, and we may take further actions from time to time as may be required by government authorities or that we determine are in the best interests of our employees, customers and business partners. Such actions could also impact our ability to fully integrate businesses we may acquire in the future. There is no certainty that such actions will be sufficient to mitigate the continuing risks posed by the virus or otherwise be satisfactory to government authorities. If significant portions of our workforce, and particularly our laboratory staff, are unable to work effectively, including due to illness, quarantines, social distancing, recruiting and retention difficulties, government actions, including the prospect of rising interest rates, inflationary pressure, and stock market volatility, or other restrictions in connection with the COVID-19 pandemic, our operations and financial results will be impacted.
The extent to which the COVID-19 pandemic will continue to impact our business, results of operations and financial condition will depend on future developments, which continue to remain highly uncertain and cannot be predicted, including, but not limited to, the continued duration and spread of the pandemic, including the contagiousness