During the third quarter of 2012, the company recorded a non-cash goodwill impairment charge of $6.9 million, equal to the total goodwill carrying amount of its United Kingdom reporting unit, which had an impact of $0.14 per diluted share. During the fourth quarter of 2011, the company recorded a non-cash goodwill impairment charge of $1.6 million resulting from its annual goodwill impairment analysis, which had an impact of $0.03 per diluted share. Adjusted operating income, adjusted net income and adjusted diluted EPS for all periods exclude goodwill impairment and are provided in this release because the company believes these amounts are useful to investors in assessing year-over-year operating performance.
Operating income for the year improved 16% to $144.9 million from $125.1 million in 2011. Adjusted operating income for 2012 increased 20% to $151.8 million compared to adjusted operating income for 2011 of $126.7 million. Adjusted operating income as a percentage of net sales (operating margin) increased to 7.8% in 2012 compared to 7.1% in 2011. Net interest expense decreased $1.5 million primarily due to the impacts of changes in estimated interest expense related to uncertain tax positions and foreign currency transaction gains and losses.
Net income increased 14% to $82.0 million in 2012 compared to $72.0 million in 2011. Earnings per share was up 16% to $1.71 per diluted share compared to $1.47 per diluted share in 2011. Adjusted net income for 2012 increased 21% to $88.9 million, while adjusted diluted EPS increased 23% to a record $1.85. Adjusted EBITDA (as defined in the addendum to this release) increased 19% to $172.9 million in 2012 compared to $145.2 million in 2011, or 8.8% of net sales in 2012 compared to 8.1% of net sales in 2011.
On the balance sheet, total net receivables increased 4% compared to December 31, 2011 due primarily to an increase in current trade receivables as a result of December base business sales growth and higher vendor receivables. Inventory levels grew 3% to $400.3 million at December 31, 2012 compared to $386.9 million at December 31, 2011, including approximately $4.0 million in inventory from recent acquisitions. Total debt outstanding at December 31, 2012 was $230.9 million, down $16.4 million from the balance at December 31, 2011.