What citrate is a common anticoagulant?
Per the NIH:


  ANTICOAGULANT CITRATE DEXTROSE A...
  ...
  Citric Acid, anhydrous, USP      0.073 g
  Sodium Citrate, dihydrate, USP   0.220 g
  Dextrose, monohydrate, USP       0.245 g
  Water for Injection, USP         q.s.
  pH: 4.5 – 5.5


They used the citrate ion in the form of citric acid and sodium citrate. The citric acid is for chleating calcium while the sodium citrate does this and acts as a buffer to prevent the citric acid from raising your blood pH too much when injected as this would be painful and hazardous. Sodium is naturally in your blood and will not interfere with action potentials and is a abundant and cheap ion to use in manufacturing so it makes a logical counter ion for the citrate buffer.