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Calcium, Ionized, Plasma / Serum

Item Value
Approval req'd? No
Available Stat? Yes
Utilization Guidelines Ionized calcium should be measured in situations where total calcium measurements may not accurately reflect the level of physiologically active free (ionized) calcium. Because equations proposed for estimating free calcium levels from measurements of total albumin and calcium are poor surrogates for true ionized calcium (1-4), direct measurements of ionized calcium should be obtained when free calcium levels are needed. Measurements of ionized calcium may be particularly useful in patients with: altered albumin concentration, acid-base disturbances, multiple myeloma, chronic kidney disease, organ transplants, borderline hypercalcemia, suspected hyperparathyroidism and normal or slightly elevated total calcium values, or symptoms of hypocalcemia despite a normal total calcium.

Note, however, that measurement of ionized calcium is significantly more labor intensive for laboratory staff than measurement of Total Calcium. In stable patients with the above abnormailities it may not be necessary to monitor the patient with repeated ionized calcium levels. In many situations determining both the ionized calcium and Total Calcium on the same sample allows the values to be compared and the patient can then be monitored with the Total Calcium alone.
Test code CAI
Test group Calcium
Performed by Parnassus, Mission Bay & Mt. Zion Chemistry
In House Availability Test available 24 hours per day 7 days per week
Method Ion selective electrode (ISE)
Parnassus: Nova 8 CRT
Mt. Zion: Gem Premier 3500
Collection Instructions Fill collection container completely. Deliver immediately to lab. Not acceptable as an "add-on" if tube has already been processed for other routine tests
Container type Light green top preferred, Gold top acceptable
Amount to Collect 2 mL blood
Click here for Microdetermination info
Sample type Plasma or serum
Preferred volume 1 mL plasma or serum
Min. Volume 0.3 mL plasma or serum
Click here for Microdetermination info
UCSF Rejection Criteria Delivered to lab > 30 min after collection
Processing notes Do not centrifuge or open tube. Deliver immediately to Chemistry. Not acceptable as an "add-on" if tube has already been processed for other routine tests
Units mmol/L
Normal range
0-1 month 1.00-1.50 mmol/L
1-6 months 0.95-1.50 mmol/L
≥ 6 months 1.16-1.36 mmol/L
Critical value < 0.80 mmol/L or > 1.55 mmol/L

Note: Panic values from Post-filter samples are not phoned.
Synonyms iCa; Free calcium; Calcium, ionized; calcium, free; Ca; Ca++
Turn around times STAT 1 hour, Routine 3 hours
Additional information Source of adult reference range: 100 UCSF blood donors (revalidated on 50 donors in March 2009).

Sources of pediatric reference ranges:

1. Snell J, Greeley C, Colaco A, et al. Pediatric reference ranges for arterial pH, whole blood electrolytes and glucose. Clin Chem 1993:39;1173 (Abstract) as referenced in "Pediatric Reference Intervals" AACC Press, 6th ed. 2007
2. J Pediatrics 114:952-956, 1989
3. European J of Pediatrics 150:464-167, 1991

NOTE – the results for serum or plasma ionized calcium are adjusted (normalized) to a standard pH of 7.40. To obtain ionized calcium results that are not adjusted to a standard pH of 7.40, order whole blood ionized calcium (e.g., in patients with possible acid-base disturbances including those with symptoms of hypocalcemia suspected to be secondary to respiratory alkalosis).

1. Ladenson JH, et al. Failure of total calcium corrected for protein, albumin, and pH to correctly assess free calcium status. J Clin Endocrinol Metab 46:986-993, 1978
2. Gauci C, et al. Pitfalls of Measuring Total Blood Calcium in Patients with CKD. J Am Soc Nephrol 19: 1592–1598, 2008
3. Bjorkman MP, et al. Calculated serum calcium is an insufficient surrogate for measured ionized calcium. Archives of Gerontology and Geriatrics, 2009
4. Thode, J. et al. Comparison of serum total calcium, albumin-corrected total calcium, and ionized calcium in 1213 patients with suspected calcium disorders. Scand. J. Clin. Lab. Invest. 49, 217–223, 1989
CPT coding 82330
LOINC code 1995-0
Last Updated 8/6/2015 7:35:50 AM
Entry Number 163
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