Study questions total calcium screening in Assaf ewes
Bottom line
A new Journal of Veterinary Internal Medicine study suggests total calcium may be an unreliable stand-in for ionized calcium when screening Assaf ewes for hypocalcemia around lambing. In 105 clinically healthy multiparous Assaf ewes from a commercial flock, researchers found total calcium had only moderate ability to detect ionized hypocalcemia before lambing, and poor performance after lambing. The study also found that ionized calcium shifted by production stage and was associated with different physiologic variables before and after lambing, including β-hydroxybutyrate in both periods. (academic.oup.com)
Why it matters: For veterinary professionals working with sheep, especially high-yielding dairy breeds, the findings reinforce that a normal or near-normal total calcium result may not tell the full story. The authors defined hypocalcemia as ionized calcium below 1.1 mmol/L and reported pre-lambing total calcium performance of AUC 0.73, with 67% sensitivity and 80% specificity at a cutoff of 2.42 mmol/L. Post-lambing, performance dropped to AUC 0.59, with 97% sensitivity but just 31% specificity at 2.77 mmol/L, a combination that could lead to substantial overclassification if total calcium is used alone. (academic.oup.com)
What to watch: Whether follow-up work in larger or more diverse flocks confirms these cutoffs, and whether on-farm protocols shift toward more direct ionized calcium testing in peri-lambing ewes. (academic.oup.com)
Key facts
- Study
- Prospective observational study
- Sample size
- 105 clinically healthy multiparous Assaf ewes
- Setting
- Single commercial flock
- Design
- Blood sampled before and after lambing
- Hypocalcemia definition
- Ionized calcium below 1.1 mmol/L
- Pre-lambing total calcium performance
- AUC 0.73, 67% sensitivity, 80% specificity at 2.42 mmol/L
- Post-lambing total calcium performance
- AUC 0.59, 97% sensitivity, 31% specificity at 2.77 mmol/L
- Main finding
- Total calcium was only moderately useful before lambing and performed poorly after lambing
- Associated variable
- β-hydroxybutyrate was associated with ionized calcium in both periods
A newly published JVIM study adds species- and breed-specific evidence to a familiar clinical problem: total calcium doesn’t reliably track the biologically active calcium fraction. In clinically healthy multiparous Assaf ewes sampled before and after lambing, researchers found total calcium was only moderately useful for identifying ionized hypocalcemia pre-lambing and performed poorly post-lambing, leading the authors to recommend prioritizing direct ionized calcium measurement. (academic.oup.com)
The question matters because calcium demand changes quickly in late gestation and early lactation, and those shifts can be especially relevant in dairy sheep. The authors note that Assaf sheep are a high-yielding dairy breed, and that early lactation can intensify calcium losses through milk production. In their cohort, ionized calcium was lower after lambing than before lambing, aligning with prior understanding that older, multiparous ewes may have less effective bone calcium mobilization during periods of high demand. Older literature has also shown that ovine hypocalcemia clusters around the weeks before lambing, underscoring how narrow the metabolic margin can be in this period. (academic.oup.com)
The study was a prospective observational analysis of 105 clinically healthy multiparous Assaf ewes from a single commercial flock. Blood was collected in the pre-lambing and post-lambing periods, with a median of 10 days before lambing for the pre-lambing sample and 18 days after lambing for the post-lambing sample. The team used a portable i-STAT 1 analyzer with CG8+ cartridges for blood gas and electrolyte testing, and defined hypocalcemia as ionized calcium below 1.1 mmol/L. (academic.oup.com)
On diagnostic performance, total calcium did not hold up especially well. Before lambing, total calcium reached an AUC of 0.73, with 67% sensitivity and 80% specificity at a cutoff of 2.42 mmol/L. After lambing, the AUC fell to 0.59; sensitivity was high at 97%, but specificity dropped to 31% at a cutoff of 2.77 mmol/L. In practical terms, that means total calcium alone may miss some pre-lambing cases and may flag too many post-lambing ewes as hypocalcemic if clinicians rely on it without direct ionized calcium data. (academic.oup.com)
The regression analysis adds nuance for clinicians interpreting calcium values around lambing. In the pre-lambing period, ionized calcium was associated with total calcium, β-hydroxybutyrate, pCO2, and pO2. Post-lambing, the associated variables shifted to total calcium, β-hydroxybutyrate, phosphorus, albumin, and sodium. That stage-specific pattern fits broader veterinary literature showing ionized calcium is influenced by more than total calcium alone, including acid-base and biochemical variables, and helps explain why a single surrogate marker can perform inconsistently across physiologic states. (academic.oup.com)
I didn’t find a separate institutional press release or substantial outside commentary on this paper yet, which isn’t unusual for a species-specific clinical research article published very recently. Still, the study’s framing is consistent with prior veterinary evidence across species that direct ionized calcium measurement is superior when accurate calcium status matters, particularly when protein status, acid-base balance, or other metabolic changes may distort the relationship with total calcium. (academic.oup.com)
Why it matters: For sheep veterinarians, consultants, and flock health teams, this is a reminder to be careful with screening shortcuts in peri-lambing animals. If a practice is using total calcium to assess subclinical hypocalcemia risk in dairy ewes, especially after lambing, the test’s low specificity in this study suggests a real risk of misclassification. That could affect treatment decisions, monitoring plans, herd-level metabolic assessments, and how veterinarians counsel pet parents or producers about transition-period disease risk. The work is also a useful prompt to think more broadly about concurrent energy balance markers, such as β-hydroxybutyrate, when interpreting calcium status. (academic.oup.com)
What to watch: The next step is external validation. Because this was a single-flock study in multiparous Assaf ewes, the most important question is whether the same performance gaps and associated variables hold in other breeds, management systems, and parity groups, including primiparous animals. If they do, direct ionized calcium testing could become harder to ignore in research protocols and higher-risk flock health workups around lambing. (academic.oup.com)