Presentation Operator Message
Hello, and welcome to the PPD Third Quarter 2021 Earnings Call and Webcast. [Operator Instructions] As a reminder, this conference is being recorded. It's now my pleasure to turn the call over to Tracy Krumme, PPD's VP and Head of Investor Relations. Please go ahead.
Tracy Krumme (Executives)
Good morning, everyone, and thank you for joining our third quarter 2021 earnings call. With me today to share our results are David Simmons, PPD's Chairman and CEO; and Chris Scully, our CFO.
Before we begin, I would like to remind everyone that our discussions include forward-looking statements that are subject to risks and uncertainties that could cause material differences in our actual results. Please refer to our 2020 annual report on Form 10-K and our most recent Form 10-Q for a discussion of these risks. With the exception of revenue, all references to income statement results are on a non-GAAP basis. I would also like to remind you of a few important details which are consistent with previous earnings calls. When referring to our financial performance, we'll be doing so on an ASC 606 basis. When referring to our commercial performance, including metrics related to net authorizations and backlog, we'll be doing so on a historical ASC 605 basis to maintain comparability with prior periods.
Lastly, due to the pending transaction between PPD and Thermo Fisher Scientific, I would like to point out that we will not be conducting a Q&A session following our prepared remarks today. And with that, I'll turn the call over to David.
David Simmons (Executives)
Thank you, Tracy, and good morning, everyone. Thank you for joining us today. I'm pleased to share that Q3 marked another strong quarter across both our segments. As I mentioned on the last call, while we look forward to the proposed merger with Thermo Fisher Scientific, it is still business as usual, which is evident in our results. To that end, I would like to thank our colleagues around the world for consistently delivering for PPD, our customers and most importantly, patients in need.
Now let me share a few third quarter highlights: First, our book of business continues to grow with year-over-year net authorizations growth of 12.7% and a net book-to-bill ratio of 1.19; second, we realized 27% revenue growth with our Clinical and Laboratory Services segments each up significantly; third, we delivered year-over-year adjusted EBITDA growth of 21%; fourth, I'd note this performance is particularly impressive against the robust growth rates we achieved in the third quarter of 2020; and finally, lastly, our balance sheet continues to strengthen as we further reduced our net leverage ratio to 3.0x as of quarter end, with our highest liquidity position on record as of September 30.
In short, the PPD story continues to be one of growth. Our relationships with existing customers continue to flourish as we remain committed to delivering on their priorities and presenting value-added solutions. Our early engagement model and track record of success continue to position us well with new customers. In addition, our expertise and capabilities that have been battle tested during the pandemic, such as in new vaccine modalities and an accelerating shift towards more decentralized and patient-centric trials are as important as ever.
To effectively prosecute our record ending backlog, our team continues to grow. We now stand at more than 30,000 colleagues globally, which represents 19% growth since September of last year. However, our focus has not just been on hiring, we are also laser-focused on retaining our talented colleagues and training and upskilling both new and existing colleagues. Several recent industry accolades highlight PPD's excellence in training and development, which make PPD a great place to work and grow one's career.
Lastly, as the drug development landscape continues to evolve even more rapidly than before with new technologies and a broader ecosystem of participants, we continue to grow and develop the differentiated capabilities that we bring to customers. As we look ahead to year-end, I'm excited about the company's trajectory and opportunities for new innovative offerings that we may be able to create as part of Thermo Fisher after we close that transaction. With that, I'll hand the call over to Chris to review our financial results.
Christopher Scully (Executives)
Thanks, David. Good morning, everyone. Similar to last quarter, I'll dive right into our results. On the commercial front, net authorizations grew 12.7% over Q3 of last year, with strong growth in both the clinical and lab segments. The continued solid performance across the business contributed to a net book-to-bill ratio of 1.19x. We closed the quarter with record ending backlog, up 20.3% year-over-year. With respect to the P&L, third quarter revenue grew 26.5% over last year, underpinned by 26% growth in clinical and 28.9% growth in labs.
Adjusted EBITDA growth for quarter 3 was 20.5%. Our revenue growth outpaced adjusted EBITDA growth as the timing of hiring and resourcing to support a robust backlog of growth temporarily impacted our adjusted EBITDA margin. However, on a year-to-date basis, excluding indirect revenue, which has generally remained higher as a percentage of total revenue for COVID studies, our growth rates for revenue and adjusted EBITDA were relatively similar. Rounding out the P&L, adjusted EPS grew 43.3% year-over-year to $0.43 a share. Please refer to our earnings release for reconciliations of our non-GAAP measures to the comparable GAAP measures.
Turning to the balance sheet. We closed the quarter with cash of $1.17 billion. With respect to liquidity, our cash balance and our available revolver capacity increased our total liquidity position to $1.77 billion as of September 30, up 60.8% over the same time last year and representing our highest ever liquidity position. Our net leverage ratio as of September 30 improved to 3.0x trailing 12-month adjusted EBITDA, down from 3.4x as of June 30 and 3.8x as of March 31.
Before wrapping up, I'll echo David's comments about being incredibly thankful to our colleagues across the organization for consistently delivering for PPD, our customers and patients in need. With a healthy industry backdrop, our strong momentum and potential opportunities to further accelerate drug development with Thermo Fisher after the closing of our merger, we're incredibly excited about our next phase of growth. And with that, operator, we'll conclude the call.
Presentation Operator Message
Thank you. That does conclude today's teleconference and webcast. You may disconnect your line at this time, and have a wonderful day. We thank you for your participation today.