West was named to the S&P 500 and recently joined the S&P 500 Dividend Aristocrats.
We had approximately 20% organic sales growth in the fourth quarter and 16% for the full year driven again by robust biologics growth, high-value product sales and contract manufacturing.
First up Q4, our financial results are summarized on Slide 11 and the reconciliation of non-U.S. GAAP measures are described in Slides 20 to 23.
We recorded net sales of $580 million, representing organic sales growth of 19.8%.
COVID-related net revenues are estimated to have been approximately $46 million in the quarter.
proprietary product sales grew organically by 25.1% 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 Q4.
We recorded a $211.1 million in gross profit, $57.9 million or 37.8% above Q4 of last year.
And our gross profit margin of 36.4% was a 390-basis-point expansion from the same period last year.
We saw improvements in adjusted operating profit with $119.1 million recorded this quarter compared to $73.1 million in the same period last year or a 62.9% increase.
Our adjusted operating profit margin of 20.5% was a 500-basis-point increase from the same period last year.
Finally, adjusted diluted earnings per share grew 63% for Q4.
Excluding stock tax benefit of $0.09 in Q4, earnings per share grew by approximately 55%.
Volume and mix contributed $87.8 million or 18.7 percentage points of growth, including approximately $46 million of volume-driven by COVID-19-related net demand.
Sales price increases contributed $5.5 million, a 1.2 percentage points of growth, and changes in foreign currency exchange rate increased sales by $16.3 million or an increase of 3.5 percentage points.
Slide 14 shows our consolidated gross profit margin of 36.4% for Q4 2020, up from 32.5% in Q4 2019.
proprietary Products fourth-quarter gross profit margin of 41.7% was 370 basis points above the margin achieved in the fourth quarter of 2019.
Contract manufacturing fourth-quarter gross profit margin of 17.2% was 80 basis points above the margin achieved in the fourth quarter of 2019.
Operating cash flow was $472.5 million for 2020, an increase of $105.3 million compared to the same period last year, a 28.7% increase.
Our 2020 capital spending was $174.4 million, $48 million higher than the same period last year and in line with guidance.
Working capital of $870.3 million at December 31, 2020, was $153.2 million higher than at December 31, 2019, primarily due to an increase in accounts receivable of $66 million due to increased sales activity and an increase in inventory of $85.6 million to position us to support the increasing needs of our customers.
Our cash balance at December 31 of $615.5 million, was $176.4 million more than our December 2019 balance, primarily due to our positive operating results.
Full year 2021 net sales guidance will be in a range of between $2.5 billion and $2.2 -- $2.525 billion.
This includes estimated net COVID incremental revenues of approximately $260 million.
There is an estimated benefit of $75 million based on current foreign exchange rate.
We expect organic sales growth to be approximately 13% to 14%.
We expect our full year 2021 reported diluted earnings per share guidance to be in a range of $6 to $6.15.
Accordingly, we have set capex guidance of $230 million to $240 million.
Estimated FX benefit on earnings per share has an impact of approximately $0.23 based on current foreign currency exchange rates and excludes future tax benefits from stock-based compensation.

Summary:
Full year 2021 net sales guidance will be in a range of between $2.5 billion and $2.2 -- $2.525 billion.
We expect organic sales growth to be approximately 13% to 14%.
We expect our full year 2021 reported diluted earnings per share guidance to be in a range of $6 to $6.15.
Accordingly, we have set capex guidance of $230 million to $240 million.