Which four components constitute the standard electrolyte panel plus a bicarbonate that is calculated?

• A. Na, K, Cl, TCO2
• B. Na, K, Cl, HCO3
• C. Na, Ca, Cl, TCO2
• D. Na, K, HCO3, TCO2

Answer: (A)

The concept tested is which components form the standard electrolyte panel and how bicarbonate is represented. In routine electrolyte testing, the four constituents are sodium, potassium, chloride, and a bicarbonate-related value. The bicarbonate is not usually measured directly in this panel; instead, it is derived from the total CO2 content of the blood. This calculated bicarbonate is reported as TCO2, so the four components you’d expect to see are Na, K, Cl, and TCO2. That’s why this option is best: it reflects the common reporting convention where bicarbonate is obtained as a calculated value (total CO2) rather than as a separately measured HCO3-. The other choices either include a non-electrolyte like calcium, omit the bicarbonate representation, or include both HCO3 and TCO2, which isn’t how the standard panel is typically documented.