Through the first four months of 2021, our participation rate in recently approved new molecular entities in the U.S. and Europe continues to be strong with over 95% of these approvals using either West or Daikyo components.
We have accelerated the timeline for capacity builds within our existing footprint by working closely with our incumbent suppliers and staging installations around the 24/7 plant schedules.
Our first phase, which began at the start of the pandemic is about 75% installed and operational, with expected completion in second half of the year.
Our financial results are summarized on Slide 9, and the reconciliation of non-U.S. GAAP measures are described in Slides 17 to 20.
We recorded net sales of $670.7 million representing organic sales growth of 31.1%.
COVID-related net revenues are estimated to have been approximately $102.9 million in the quarter.
Proprietary products sales grew organically by 39.6% in the quarter.
High-value products, which made up more than 70% of proprietary products sales in the quarter grew double digits and had solid momentum across all market units throughout Q1.
We recorded $271.9 million in gross profit, $104.9 million or 62.8% above Q1 of last year.
And our gross profit margin of 40.5% was a 650-basis-point expansion from the same period last year.
We saw improvements in adjusted operating profit with $179.2 million recorded this quarter, compared to $88 million in the same period last year, for a 103.6% increase.
Our adjusted operating profit margin of 26.7% was an 880-basis-point increase from the same period last year.
Finally, adjusted diluted earnings per share grew 103% for Q1.
Excluding stock-based compensation tax benefit of $0.15 in Q1, earnings per share grew by approximately 102%.
Volume and mix contributed $146.7 million or 29.8 percentage points of growth, including approximately $102.9 million of volume driven by COVID-19-related net demand.
Sales price increases contributed $6 million or 1.2 percentage points of growth, and changes in foreign currency exchange rates increased sales by $26.5 million or an increase of 5.4 percentage points.
Slide 12 shows our consolidated gross profit margin of 40.5% for Q1 2021, up from 34% in Q1 2020.
Proprietary products first-quarter gross profit margin of 46.3% was 610 basis points above the margin achieved in the first quarter of 2020.
The key drivers for the continued improvement in proprietary products gross profit margin were favorable mix of products sold driven by growth in high-value products, production efficiencies, one-time fees associated with certain canceled COVID supply agreements of approximately $11.8 million, and sales price increases, partially offset by increased overhead costs, inclusive of compensation.
Contract manufacturing first-quarter profit gross margin of 15.7% was 140 basis points above the margin achieved in the first quarter of 2020.
Operating cash flow was $88.7 million for the first quarter of 2021, an increase of $31.6 million compared to the same period last year or a 55.3% increase.
Our first-quarter 2021 capital spending was $54.7 million, $22.6 million higher than the same period last year and in line with guidance.
Working capital of $844.2 million at March 31, 2021, declined slightly by $26.1 million from December 31, 2020.
Our cash balance at March 31 of $483.7 million was $131.8 million less than our December 2020 balance primarily due to our share repurchase program activity offset by the positive operating results.
Full-year 2021 net sales are expected to be in a range of $2.63 billion and $2.655 billion, compared to prior guidance range of $2.5 billion and $2.525 billion.
This guidance includes estimated net COVID incremental revenues of approximately $345 million.
There is an estimated benefit of $75 million based on current foreign exchange rates.
We expect organic sales growth to be approximately 19% to 20%.
We expect our full-year 2021 adjusted diluted earnings per share guidance to be in a range of $6.95 to $7.10, compared to a prior range of $6 to $6.15.
We are keeping our capex guidance at $230 million to $240 million but continue to evaluate the levels needed to support our continued growth.
Estimated FX benefit on earnings per share has an impact of approximately $0.23 based on current foreign currency exchange rates.

Summary:
We recorded net sales of $670.7 million representing organic sales growth of 31.1%.
Full-year 2021 net sales are expected to be in a range of $2.63 billion and $2.655 billion, compared to prior guidance range of $2.5 billion and $2.525 billion.
We expect our full-year 2021 adjusted diluted earnings per share guidance to be in a range of $6.95 to $7.10, compared to a prior range of $6 to $6.15.