The MLB and MLBPA jointly released a summary report of drug tests performed since 2007, stating that 10,955 took place from the start of the 2007 offseason through the beginning of this year.
The breakdown is as follows:
3,486 tests in 2008
3,722 tests in 2009 (121 in the offseason)
3,747 tests in 2010 (138 in the offseason)
I certainly wouldn’t have guessed that there were so few offseason tests. With somewhere around 1,000-1,100 players on 40-man rosters eligible to be tested, that means barely one out of every eight players is getting tested over the winter.
On the other hand, it looks like everyone is getting tested multiple times during the season.