Revenue for the first quarter grew 24% on a reported basis and 21% at constant currency was $209 million above the high end of our guidance range, but about half of this beat came from strong operational performance and half was from higher pass-throughs.
First quarter adjusted EBITDA grew 32%, reflecting our revenue growth and productivity measures.
The $69 million beat above the high end of our guidance range was entirely due to the stronger organic revenue performance.
First quarter adjusted diluted earnings per share of $2.18 grew 45%.
During the quarter, a top 10 pharma client deployed our next best action solution in 14 countries.
We added another 10 new clients this quarter and now have 150 clients deploying about 70,000 users.
To-date, we've been awarded over 125 studies with over 300,000 patients enrolled and over 4 million surveys completed.
In the first quarter, we achieved a contracted net book-to-bill ratio of 1.41, including pass-throughs and 1.41 excluding pass-throughs.
At March 31st, our LTM contracted book-to-bill ratio was 1.52 including pass-throughs and 1.45 excluding pass-throughs.
Our contracted backlog in R&DS including pass-throughs grew 18.3% year-over-year to $23.2 billion at March 31, 2021.
As a result, our next 12 months revenue from backlog increased by over $600 million sequentially to $6.5 billion, that's up 31.1% year-over-year.
We are working with 5 of the top 10 pharma client and to-date we've recruited almost 170,000 patients using our advanced decentralized trial solutions.
First quarter revenue of $3,409 million grew 23.8% on a reported basis.
Analytics Solutions revenue for the first quarter was $1,348 million, which was up 20.7% reported and 17.1% at constant currency.
R&D Solutions first quarter revenue of $1,868 million improved 29.6% at actual FX rates, and 28.1% at constant currency.
Pass-through revenues were a tailwind of 770 basis points to the R&DS revenue growth rate in the quarter.
CSMS revenue of $193 million was down 1.5% reported and 4.1% on a constant currency basis.
Moving down to P&L, adjusted EBITDA was $744 million for the quarter.
Margins expanded 140 basis points despite significant headwinds from higher pass-through revenue and lower margin COVID work.
GAAP net income was $212 million and GAAP diluted earnings per share were $1.09.
Adjusted net income was $425 million for the first quarter and adjusted diluted earnings per share grew 45.3% between [Technical Issues] $2.18.
Backlog was up 18.3% year-over-year to $23.2 billion at March 31.
Next 12 months revenue as Ari mentioned from backlog grew significantly and currently stands at $6.5 billion, up 31.1% year-over-year.
At March 31, cash and cash equivalents totaled $2.3 billion and debt was $12.2 billion, resulting in net debt of $9.9 billion.
Our net leverage ratio at March 31 improved to 3.9 times trailing 12 month adjusted EBITDA, marking the first time since just following the merger that this ratio was below 4 times.
And this is particularly noteworthy, you may recall that in 2019, when we gave you our three year guidance, we committed to delever to 4 turns or below exiting 2022.
Cash flow from operations was $867 million, capex was $149 million, resulting in free cash flow of $718 million.
We repurchased $50 million of our shares in the quarter, which leaves us with $867 million of share repurchase authorization remaining under the program.
You'll recall that back on April 1, when we announced the acquisition of Quest 40% interest in our Q Squared joint venture, we raised our 2021 earnings per share guidance by $0.12 to reflect the elimination of Quest minority interest in the joint venture's earnings.
We're raising our full-year 2021 revenue guidance, both at the low and high end of that range, resulting in an increase of $625 million at the midpoint of the range.
The new revenue guidance is $13,200 million to $13,500 million, which represents year-over-year growth of 16.2% to 18.8%.
Now compared to the prior year, FX is expected to be a tailwind of about 150 basis point to full-year revenue growth.
We're also raising our full-year profit guidance as a result of stronger revenue outlook, we've increased it, increased adjusted EBITDA guidance at both the low and high end of the range, resulting in an increase of $133 million at the midpoint.
Our new full-year guidance is $2,900 million to $2,965 million, which represents year-over-year growth at 21.6% to 24.4%.
Moving to EPS, I mentioned Q Squared transaction on April 1, as a result of that, we raised our adjusted diluted earnings per share guidance by $0.12 to a new range of $7.89 to $8.20.
We're now raising both the low and the high end of that guidance range, resulting in a new adjusted diluted earnings per share guidance of $8.50 to $8.75 or year-over-year growth of 32.4% to 36.3%.
Moving to detail on P&L, interest expense is expected to be approximately $400 million for the year, operational depreciation and amortization is still expected to be somewhat over $400 million and we're continuing to assume an effective tax rate of approximately 20% for the full year.
Now let's turn to the second quarter guidance, assuming FX rates remain constant through the end of the quarter, second quarter revenue is expected to be between $3,225 million and $3,300 million, which represents reported growth of 27.9% to 30.9%.
Adjusted EBITDA is expected to be between $690 million and $715 million, which represents reported growth of 42.9% to 48%.
And finally, adjusted diluted earnings per share is expected to be between $2 and $2.10, up 69.5% to 78%.
This included revenue growth of over 20% in both our TAS and R&DS segment.
R&DS backlog improved to $23.2 billion, up 18% year-over-year.
Next 12 months revenue from that backlog increased to $6.5 billion, up 31% year-over-year.
Net leverage improved to 3.9 times trailing 12 month adjusted EBITDA.

Summary:
First quarter adjusted diluted earnings per share of $2.18 grew 45%.
First quarter revenue of $3,409 million grew 23.8% on a reported basis.
GAAP net income was $212 million and GAAP diluted earnings per share were $1.09.
Adjusted net income was $425 million for the first quarter and adjusted diluted earnings per share grew 45.3% between [Technical Issues] $2.18.
The new revenue guidance is $13,200 million to $13,500 million, which represents year-over-year growth of 16.2% to 18.8%.
We're now raising both the low and the high end of that guidance range, resulting in a new adjusted diluted earnings per share guidance of $8.50 to $8.75 or year-over-year growth of 32.4% to 36.3%.
Now let's turn to the second quarter guidance, assuming FX rates remain constant through the end of the quarter, second quarter revenue is expected to be between $3,225 million and $3,300 million, which represents reported growth of 27.9% to 30.9%.
And finally, adjusted diluted earnings per share is expected to be between $2 and $2.10, up 69.5% to 78%.