We had 14% organic sales growth in the second quarter driven, again, by robust high-value product sales.
GAAP measures are described in Slides 13 to 17.
We recorded net sales of $527.2 million, representing organic sales growth of 14.3%.
COVID-related net revenues are estimated to have been approximately $19 million in the quarter.
We recorded $195.1 million in gross profit, $37.2 million or 23.6% above Q2 of last year.
And our gross profit margin of 37% was a 340-basis-point expansion from the same period last year.
We saw improvement in adjusted operating profit, with $106 million recorded this quarter, compared to $81.9 million in the same period last year for a 29.4% increase.
Our adjusted operating profit margin of 20.1% was a 270-basis-point increase from the same period last year.
Finally, adjusted diluted earnings per share grew 40% for Q2.
Excluding stock tax benefit of $0.09 in Q2, earnings per share grew by approximately 38%.
Our proprietary product sales grew organically by 13.3% in the quarter.
High-value products, which made up more than 65% of proprietary product sales in the quarter, grew double digits and had solid momentum across all market units throughout Q2.
Volume and mix contributed $59.4 million or 12.6 percentage points of growth, including approximately $19 million of volume driven by COVID-19-related net demand.
Sales price increases contributed $7.8 million or 1.7 percentage points of growth, and changes in foreign currency exchange rates reduced sales by $9.6 million or a reduction of 2 percentage points.
Slide 9 shows our consolidated gross profit margin of 37% for Q2 2020, up from 33.6% in Q2 2019.
Proprietary products' second-quarter gross profit margin of 42.8% was 330 basis points above the margin achieved in the second quarter of 2019.
Contract manufacturing second-quarter gross profit margin of 19% was 470 basis points above the margin achieved in the second quarter of 2019.
There was approximately 180 to 200 basis points positive impact on margin, primarily due to a onetime engineering project work.
Our adjusted operating profit margin of 20.1% with a 270-basis-point increase from the same period last year, largely attributable to our gross profit expansion.
One point to note, we took a onetime charge of $6.3 million for asset impairment.
Operating cash flow was $205.2 million for the year-to-date 2020, an increase of $52.5 million, compared to the same period last year, a 34% increase.
Our year-to-date capital spending was $69.2 million, $12.1 million higher than the same period last year and in line with guidance.
Working capital of $735.4 million at June 30, 2020, was $18.3 million higher than at December 31, 2019, primarily due to an increase in inventory, mainly as a result of increasing our safety stock levels and accounts receivable due to increased sales activity.
Our cash balance at June 30 of $445.9 million was $6.8 million more than our December 2019 balance, primarily due to our positive operating results.
Full-year 2020 net sales guidance will be in a range of between $2.035 billion and $2.055 billion.
This includes estimated net COVID incremental revenues of $60 million.
There is an estimated headwind of $26 million based on current foreign exchange rates.
We expect organic sales growth to be approximately 12.5%.
This compares to prior guidance of $1.95 billion to $1.97 billion and growth of 8%.
We expect our full-year 2020 reported diluted earnings per share guidance to be in a range of $4.15 to $4.25, compared to prior guidance of $3.52 to $3.62.
Capex guidance has raised to $170 million to $180 million.
This compares to previous guidance of $130 million to $140 million.
Estimated FX headwind on earnings per share has an impact of approximately $0.07 based on current foreign currency exchange rates.
The revised guidance also includes $0.16 earnings per share impact from our H1 tax benefits from stock-based compensation.
Our long-term construct remains at approximately 6% to 8% organic sales growth and continued earnings per share expansion.

Summary:
We recorded net sales of $527.2 million, representing organic sales growth of 14.3%.
Full-year 2020 net sales guidance will be in a range of between $2.035 billion and $2.055 billion.
There is an estimated headwind of $26 million based on current foreign exchange rates.
We expect our full-year 2020 reported diluted earnings per share guidance to be in a range of $4.15 to $4.25, compared to prior guidance of $3.52 to $3.62.