Our robust first quarter financial performance, highlighted by 13% organic revenue growth and 170 basis points of year-over-year operating margin improvement demonstrates the strength of the biopharmaceutical market environment and the power of our unique portfolio, both of which we believe are as strong as they have ever been.
Organic revenue was about 10% for a second consecutive quarter, even after normalizing for last year's COVID-19 impact.
Quarterly revenue surpassed $800 million for the first time and an $824.6 million for the first quarter of 2021 represented a 16.6% increase over last year.
Organic revenue growth of 13% was driven by double-digit growth across all three segments.
The year-over-year comparison to last year's COVID-related revenue impact, which primarily affected the RMS segment, contributed approximately 140 basis points to the revenue growth rate this quarter.
The operating margin was 20.7%, an increase of 170 basis points year-over-year.
We expect the same factors will drive margin improvement for the year and believe the operating margin will approach 21%, above our prior target.
Earnings per share were $2.53 in the first quarter, an increase of 37.5% from $1.84 in the first quarter of last year.
We now expect organic revenue growth in a range of 12% to 14%, a 300 basis point increase from our prior range.
Non-GAAP earnings per share are expected to be $9.75 to $10, which represents 20% to 23% year-over-year growth and an increase of $0.75 at the midpoint from our prior outlook.
Revenue was $501.2 million in the first quarter, an 11.6% increase on an organic basis over the first quarter 2020 driven by broad-based demand for both Discovery and Safety Assessment.
The DSA operating margin increased by 180 basis points to 23.8% in the first quarter.
Leverage from the robust DSA revenue growth was the primary driver of margin improvement, and we expect this trend will continue to propel the DSA margin into the mid-20% range for the year.
RMS revenue was $176.9 million, an increase of 14.8% on an organic basis over the first quarter of 2020.
Approximately 620 basis points of the increase was attributable to the comparison to last year's COVID-related revenue impact from client site closures and disruption.
In the first quarter, the RMS operating margin increased 570 basis points to 28.7%.
First, last year's 23% margin was depressed by the onset of COVID-related client disruptions and the resulting impact on the research model order activity.
Revenue for the Manufacturing segment was $146.5 million, a 15.6% increase on an organic basis over the first quarter of last year.
The Manufacturing segment's first quarter operating margin was stable at 35.5%.
This is consistent with the historical trend in the first quarter and in line with our revised expectations in 2021 for a mid-30% operating margin when factoring in the Cognate acquisition.
Microbial Solutions growth rate rebounded above the 10% level in the first quarter, reflecting strong demand for our Endosafe endotoxin testing systems, cartridges and core reagents for all geographic regions.
Now David Smith will give you additional details on our first quarter results and 2021 guidance.
We delivered strong revenue growth, well above the 10% level on an organic basis and significant operating margin expansion of 170 basis points, which drove earnings-per-share growth of 37.5% to $2.53.
We now expect to deliver reported revenue growth of 19% to 21% and organic revenue growth in a range of 12% to 14% for the full year.
Given the robust top line performance, we expect to drive meaningful operating margin improvement this year with the full year margin approaching 21%.
This is expected to drive better-than-expected earnings per share in a range of $9.25 to $10, which represents year-over-year growth above 20%.
Including the acquisition of Cognate, Manufacturing's reported revenue growth rate is expected to be in the high-30% range.
With regard to operating margin, RMS will continue to be a primary contributor to the overall improvement for the year, with the segment margin meaningfully above 25%.
We also expect the DSA segment operating margin to increase over the prior year into the mid-20% range.
When factoring in Cognite, the Manufacturing segment's operating margin is expected to be in the mid-30% range this year or moderately below its 2020 level.
Unallocated corporate costs was slightly higher than our expectations, totaling 6.2% of total revenue or $51.2 million in the first quarter compared to 5.6% of revenue in the first quarter of last year.
Despite the higher expenses in the first quarter, we continue to expect unallocated corporate costs to be in the mid-5% range as a percentage of revenue for the full year.
The first quarter tax rate was 14.5%, a 20 basis point increase year-over-year and consistent with our outlook in February, which calls for a tax rate in the mid-teens due to the gating of the excess tax benefit from stock-based compensation.
We continue to expect our full year tax rate will be in the low-20% range on a non-GAAP basis, which is unchanged from our outlook provided in February.
Total adjusted net interest expense for the first quarter was $17.1 million, which was essentially flat sequentially and a decrease of nearly $2 million year-over-year due to lower average debt levels, which resulted in interest rate savings based on our leverage ratio.
At the end of the first quarter, we had $2.2 billion of outstanding debt, representing a gross leverage ratio of 2.3 times and a net leverage ratio of 1.9 times.
In March, we issued $1 billion of senior notes to further optimize our capital structure and take advantage of the attractive interest rate environment.
The proceeds of this bond offering we used to redeem a previously issued higher-rate $500 million bond to pay down the existing term loan and a portion of the revolving credit facility and to finance a portion of the Cognate acquisition.
In April, we also amended our existing credit agreement to establish a new revolver with borrowing capacity of up to $3 billion.
The net result of these actions will reduce our average interest rate on debt by approximately 50 basis points to 2.65%.
On a pro forma basis, including the Cognate and Retrogenix acquisitions, our gross leverage ratio was just under three times, and we had total debt outstanding of slightly below $3 billion.
For the year, the higher debt balances due primarily to Cognate acquisition will be partially offset by the lower average interest rate from these refinancing activities, which is expected to result in total adjusted net interest expense of $83 million to $86 million.
Free cash flow was $142.2 million in the first quarter, a significant increase compared to $42.9 million last year.
Capital expenditures were $28 million in the first quarter compared to $25.7 million last year.
Looking ahead, we are increasing our capex guidance for 2021 by $40 million to approximately $220 million.
Even with the additional capital, we expect capex will remain below 7% of our total revenue this year, which is consistent with the target that we provided at our last Investor Day in 2019.
For the full year, we are updating our free cash flow guidance to the upper end of the prior range and now expect free cash flow of approximately $435 million for the full year.
We now expect second quarter reported revenue growth at or near the 30% level, including the contribution of Cognate.
On an organic basis, we expect second quarter growth rate to be at or near 20%.
This reflects the prior year comparison to the COVID-related revenue impact, which will contribute approximately 700 basis points to the second quarter revenue growth.
Our expectation for non-GAAP earnings per share is a growth rate of more than 50% year-over-year.

Summary:
Quarterly revenue surpassed $800 million for the first time and an $824.6 million for the first quarter of 2021 represented a 16.6% increase over last year.
Earnings per share were $2.53 in the first quarter, an increase of 37.5% from $1.84 in the first quarter of last year.
We now expect organic revenue growth in a range of 12% to 14%, a 300 basis point increase from our prior range.
Non-GAAP earnings per share are expected to be $9.75 to $10, which represents 20% to 23% year-over-year growth and an increase of $0.75 at the midpoint from our prior outlook.
Now David Smith will give you additional details on our first quarter results and 2021 guidance.
We delivered strong revenue growth, well above the 10% level on an organic basis and significant operating margin expansion of 170 basis points, which drove earnings-per-share growth of 37.5% to $2.53.
We now expect to deliver reported revenue growth of 19% to 21% and organic revenue growth in a range of 12% to 14% for the full year.