In one sentence
Your blood calcium adjusted for the albumin that carries about half of it, so that a low or high albumin does not make the calcium look low or high when the free, active calcium is unchanged.
Why it matters
Calcium in the blood is held within a narrow band by the parathyroid glands, vitamin D and the kidneys, and most of what a laboratory measures is bound to albumin rather than free. In 200 consecutive specimens sent for liver tests, calcium tracked albumin closely (a correlation of 0.87), a quarter of the samples read low on calcium only because their albumin was low, and a simple adjustment on albumin brought the spread back to the laboratory's reference range (Payne 1973). For a well member the figure matters in one situation the clinician specification names: taking vitamin D with calcium, where a high corrected calcium is the thing a doctor would want to see.
What moves it
Albumin, which the adjustment removes; dehydration and a tight tourniquet, which concentrate both; vitamin D and calcium supplements; the parathyroid glands; and, rarely, conditions a doctor investigates. A laboratory may print its own adjusted calcium using its own equation, which VT shows first.
How to measure it well
Calcium and albumin from one draw, taken under the panel's rules, with calcium supplements recorded. Not computed when the laboratory did not bundle calcium with the electrolytes, which many New Zealand request forms do not.
How Vitality Tracker uses it
Computed only when calcium and albumin come from one collection, shown in the kidney and electrolytes panel with the laboratory's own adjusted value first where it prints one, and against the laboratory's range alone; VT draws no working line of its own (the optimal set v0.1 says lab). When vitamin D and calcium are on the Supplement Tracker, the figure is shown beside the vitamin D result.
How far to trust the number
Calculated from two precise results; the adjustment is an approximation derived in one laboratory's population fifty years ago, and laboratories use their own versions, which is why the laboratory's figure comes first.
Sources
- Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J 1973;4:643–6. doi:10.1136/bmj.4.5893.643
- Vitality Tracker, Clinician report content specification (derived indices).