The lavender EDTA tube binds calcium; if it's drawn before an SST tube, the calcium values could test lower than accurate. Which of the following statements is true?

• A. False
• B. True
• C. Calcium values would test higher than accurate
• D. Calcium values would be unaffected

Answer: (B)

Calcium measurements can be affected by carryover of the tube’s additive from a previous collection. EDTA in the lavender tube binds calcium, so if that tube is drawn before a serum-separator tube and some EDTA contaminates the subsequent sample (through the needle, syringe, or the line not being cleared), calcium in the SST sample would be chelated and reported as lower than its true value. That’s why the statement is true: drawing the EDTA tube before the SST can cause calcium values to test lower than accurate. If there’s no contamination, calcium would be unaffected, and the other possibilities don’t apply.