Thank you for standing by. My name is Dee, and I will be your conference operator. At this time, I would like to welcome everyone to the Privia Health First Quarter conference call. All lines have been placed on mute to prevent any background noise. After the speaker's remarks, there will be a question-and-answer session. If you would like to ask a question during this time, simply press star, followed by the number one on your telephone keypad. If you would like to withdraw your question, press star one again. Thank you. I would like to turn the call over to Robert Borchert, Senior Vice President of Investor and Corporate Communications. Please go ahead, sir.
Thank you, Dee. Good morning, everyone. Joining me are Parth Mehrotra, our Chief Executive Officer, and David Mountcastle, our Chief Financial Officer. This call is being webcast and will be accessed through the Investor Relations section of priviahealth.com, along with today's financial press release and slide presentation. Following our prepared comments, we'll open the line for questions. Please limit yourself to one question only and return to the queue if you have a follow-up, so we can get to as many questions as possible today. The financial results reported today are preliminary and are not final until our Form 10-Q for the first quarter ended March 31, 2025, is filed with the Securities and Exchange Commission. Some statements we will make today are forward-looking in nature based on our current expectations and view of our business as of May 8, 2025.
Such statements, including those related to our future financial and operating performance and future business plans and objectives, are subject to risks and uncertainties that may cause actual results to differ materially. As a result, these statements should be considered along with the cautionary statements in today's press release and the risk factors described in our company's most recent SEC filings. Finally, we may refer to certain non-GAAP financial measures on the call, and reconciliation of these measures to comparable GAAP measures are included in our press release and the accompanying slide presentation posted on our website. Now I'd like to hand the call over to our CEO, Parth Mehrotra.
Thank you, Robert, and good morning, everyone. Privia Health started 2025 with strong growth and momentum as we continue to execute very well across all aspects of our business. This morning, I'll cover our first quarter performance and recent business highlights. Then David will discuss our market presence, financial results, and our 2025 guidance update before we take your questions. Privia's growth team has continued to deliver strong new provider signings across all of our markets, which underpins our visibility through 2025. Implemented provider growth of 11.7% and value-based attribution growth of 11.1% year-over-year helped drive total practice collections growth of 12.8%. Adjusted EBITDA increased 35.1%, with EBITDA margin expanding 460 basis points year-over-year while we continue to invest in growth and expansion. This highlights the scale and strength of our business model.
In early April, we announced our entry into the state of Arizona as we pursue disciplined growth to complement our organic sales engine. Following this outstanding overall performance in the first quarter, we are raising our 2025 outlook to the mid to high end of our initial guidance. Attributed Lives guidance remains unchanged. This guidance raised early in the year is being driven by our excellent operating execution and continued exceptional performance by all of our provider partners in the current healthcare services environment. We announced our partnership in early April with IMS as our anchor practice to enter the state of Arizona. IMS is one of the largest independent multi-specialty practices in Arizona with approximately 70 providers. This is a very strategic market entry for Privia as the demographics of Arizona offer a compelling value-based care opportunity.
IMS has more than 28,000 value-based care attributed lives across commercial, Medicare, and Medicaid programs. The transaction value was $95 million at closing. Privia Health owns Medical Group Entity and the Management Services Organization in Arizona. IMS remains physician-owned and will operate with significant clinical autonomy. We expect the Arizona market to be EBITDA positive in the fourth quarter this year following IMS's implementation on the Privia platform. We also expect it to meaningfully contribute to adjusted EBITDA in 2026, giving us confidence in our ability to continue to target 20% EBITDA growth. We look forward to building on the IMS culture of delivering high-quality community care as we continue to align with like-minded providers across the United States. Now I'll ask David to review our market position, recent financial results, balance sheet, and discuss our 2025 guidance outlook in more detail.
Thank you, Parth. With the addition of Arizona, Privia Health now operates across 15 states and the District of Columbia. Our footprint of high-quality community-based medical groups and risk entities comprises 4,871 implemented providers caring for over 5.2 million patients in more than 1,200 care center locations. Privia now serves 1.27 million attributed lives across more than 100 commercial and government value-based care programs. This breadth and geographic reach positions us as a highly diversified and balanced value-based care organization. Total attributed lives at March 31 increased 11.1% from a year ago. This was driven by new provider growth as well as new value-based care contracts in certain programs. Commercial attributed lives increased 13.6% from last year to reach 779,000. Lives attributed to the Medicare Shared Savings Program were up almost 5%. Medicare Advantage and Medicaid attribution increased more than 8% and 11% respectively from a year ago.
The diversification of Privia's value-based care contracts gives us confidence in our ability to build scale and profitability across the business despite challenges in any one particular program or contract. We remain highly focused on generating positive contribution margin in our value-based care contracts as we pursue attribution growth, manage risk, and implement clinical and operational enhancements in our partner practices. We continue to evaluate moving to greater downside risk when we believe contract economics appropriately compensate our physician partners for the level of risk incurred. Ultimately, our goal is to achieve consistent and sustainable earnings growth for our medical groups and shareholders. Privia Health executed very well to start 2025 with strong growth and momentum across all our markets. Implemented providers grew 82 sequentially from Q4 to reach 4,871 at March 31st, an increase of 11.7% year-over-year.