First, cash flow. Our cash flow remains strong. Excluding the Hunter Lab settlement of $49.5 million, free cash flow for the year ended December 31, 2011, was $759.4 million. Cash flow has been negatively impacted by approximately $17 million, due to delays in the Genzyme Genetics enrollment process, which we expected and have previously discussed. The impact from these delays has improved by $11 million since the third quarter of 2011, and we expect to continue to make progress in resolving these issues. We remain pleased with our cash collections. DSO was 46 days at the end of December, unchanged year-over-year and sequentially. Our bad debt remained 4.5%.
Second, revenue growth. Revenue increased 5.5% year-over-year in the fourth quarter. Genzyme Genetics accounted for approximately 4.8% of this growth. During the quarter, revenue per requisition increased 4.2% year-over-year. Genzyme Genetics accounted for approximately 4.1% of the growth in revenue per requisition. Total company volume increased 1.2% year-over-year during the fourth quarter. Genzyme Genetics accounted for approximately 0.6% of this volume growth. Esoteric volume increased approximately 3.5% in the quarter.
Third, margin. For the fourth quarter our adjusted operating income margin was 18.9% compared to 19.5% in the fourth quarter of 2010. Year-over-year margin decline was due entirely to recent acquisitions that we have not fully integrated. Excluding these acquisitions, margins would have remained stable year-over-year. Margins will improve as we integrate these businesses and our guidance for next year implies margin expansion.
Fourth, liquidity. We remain well-capitalized. At the end of December, we had cash of $159.3 million and approximately $400 million available under our revolving line of credit. During the fourth quarter, we replaced existing credit facilities with a new $1 billion revolver. We borrowed $560 million under the new facility and repaid all amounts outstanding under our previous term loan and revolving credit facility. In addition to the cash generated by operations, we believe the new $1 billion revolver will provide us with ready liquidity to carry out strategic initiatives. At the end December, total debt was $2.2 billion.Read the rest of this transcript for free on seekingalpha.com