Concentrations of Credit Risk
Financial instruments that potentially subject us to concentrations of credit risk consist primarily of cash and cash equivalents, derivative financial instrument contracts and accounts and notes receivable. Our investment policy limits exposure to concentrations of credit risk and changes in market conditions. Counterparties to financial instruments expose us to credit-related losses in the event of nonperformance. We transact our financial instruments with a diversified group of major financial institutions with investment grade credit ratings and actively monitor their credit ratings and our outstanding positions to limit our credit exposure. We provide credit, in the normal course of business, to hospitals, healthcare agencies, clinics, doctors' offices and other private and governmental institutions and generally do not require collateral. We record our accounts receivable in our consolidated balance sheets at net realizable value. We perform on-going credit evaluations of our customers and maintain allowances for potential credit losses, based on historical information and management's best estimates. Amounts determined to be uncollectible are written off against this reserve. We recorded write-offs of uncollectible accounts receivable of $12 million in 2013, $7 million in 2012, and $13 million in 2011. We are not dependent on any single institution and no single customer accounted for more than ten percent of our net sales in 2013, 2012, or 2011 or accounts receivable at December 31, 2013 or 2012; however, large group purchasing organizations, hospital networks and other buying groups have become increasingly important to our business and represent a substantial portion of our U.S. net sales.
We closely monitor outstanding receivables for potential collection risks, including those that may arise from economic conditions, in both the U.S. and international economies. Our European sales to government-owned or supported customers in Southern Europe, specifically Greece, Italy, Spain and Portugal are subject to an increased number of days outstanding above historical levels prior to payment. Historically, receivable balances with certain publicly-owned hospitals in these countries accumulate over a period of time and are then subsequently settled as large lump sum payments. While we believe our allowance for doubtful accounts in these countries is adequate as of December 31, 2013, if significant changes were to occur in the payment practices of these European governments or if government funding becomes unavailable, we may not be able to collect on receivables due to us from these customers and our write-offs of uncollectible amounts may increase. As of December 31, 2013, our net receivables in these countries greater than 180 days past due totaled approximately $95 million, of which approximately $50 million were past due greater than 360 days.
