Understanding calcium in poultry: what producers need to know

While over 99% of a bird's calcium is stored in its bones, it’s the remaining 1% that could make or break your flock's health. Understanding and managing this tiny fraction of calcium can significantly impact your animals’ productivity and well-being. Verax^TM helps you unlock this potential by providing insights into calcium management.

Why Calcium Matters

Calcium is an essential mineral for birds, involved in several important biological processes. In blood, calcium is found in three separate fractions: protein-bound, anion-bound, and free (often referred to as ionized). In a healthy bird, the sum of all fractions (total blood calcium) provides an accurate perspective of their calcium status. However, the amount of calcium found in each of the three fractions varies depending on the blood concentration of hydrogen ions (blood pH), anions, and plasma proteins (especially albumin). Any disruption to these blood features can seriously influence the relevance of the total calcium value.

More than 99% of all body calcium in birds (and indeed in mammals) is found in the skeleton. The remaining 1% is distributed between the extracellular and intracellular pools. Intracellular calcium is involved in cell division, muscle contraction, glycogen metabolism, and other important functions. Circulating extracellular calcium is required for bone mineralization, blood coagulation, and as a cofactor for various enzymes. This circulating extracellular calcium is what is measured as total blood calcium, and the normal range for broiler chickens is around 11.5-12.5 mg/dl. However, the circulating extracellular calcium exists in three distinct forms: ionized calcium (approximately 47%), protein-bound (approximately 48%), and anion-bound (around 5%).

Types of Calcium in Blood

The major plasma protein involved in calcium binding is albumin, and around 80% of the protein-bound calcium is covalently associated with this protein. The remaining 20% of the protein-bound calcium is associated with globulin. Each molecule of albumin has approximately 30 calcium binding sites. The relatively small proportion of calcium that is anion-bound is associated with phosphate, lactate, citrate, and various free fatty acids. It should be noted that while this is typically only 5-10% of total calcium, it may be higher if, for example, blood lactate concentrations are unusually high following strenuous activity. This latter effect may occur if birds are stressed during handling and engage in prolonged wing flapping, especially locally to the site of the blood draw. Such an effect may be detectable in the blood anion gap.

Technically, protein-bound, anion-bound, and free blood calcium are all ionized. However, in most cases, the term ionized is restricted as a descriptor of free circulating extracellular calcium. In birds, around 47% of the total calcium is free or ionized, and it is this calcium pool that is metabolically active. Additionally, it is the ionized calcium that binds to the calcium-sensing receptors and so is tightly regulated by the parathyroid hormone and 1,25 hydroxyvitamin D.

Insights with Verax^TM

With Verax^TM, total blood calcium is measured via the VetScan 2 device, which delivers values in mg/dl, with normal plasma concentrations in the 11.5-12.5 mg/dl range. Ionized calcium is measured via the iSTAT Alinity, and typical values are in the 1.30-1.40 mmol/l range. Total calcium is the sum of all other calcium fractions, including the ionized calcium, so each of these separate fractions exist in equilibrium.

If ionized calcium increases, then one of the other fractions must be reduced. As we do not specifically measure anion-bound or protein-bound calcium, we must infer changes in these based on the percentage of calcium that is ionized and unusual presentation of blood albumin, pH, or the unmeasured anion gap (the sum of Na and K, minus the sum of HCO3 and Cl).

Clinical Significance

Total calcium, in many cases, is an accurate indicator of the calcium status of the bird. However, if circulating protein concentration is abnormal or the bird is suffering from metabolic or respiratory acid/base disturbance, total calcium becomes unreliable as a metric to describe calcium homeostasis.

For example, plasma albumin may be reduced in birds with hepatic pathologies such as mycotoxins or if dermatological issues exist such as with NH3 burns. A drop in plasma albumin would result in a decrease in total blood calcium. Although there are equations that can correct total blood calcium for variance in plasma protein concentration, these are subject to some serious limitations, so direct measurement of ionized calcium is preferred.

The proportion of calcium in blood that is free or ionized is very sensitive to changes in pH, and the concentration of ionized calcium, at a given plasma protein concentration, will change by around 2-4% for every 0.1 unit change in blood pH. The reason for this relationship is that hydrogen ions compete with calcium for binding sites on both albumin and globulin, and so as hydrogen ion concentration increases (low pH), the plasma proteins lose their capacity to sequester calcium, returning it to the ionized pool.

In birds, while around 47% of total blood calcium is ionized in a normal, healthy individual, this can vary from around 43-53% and even more in cases of extreme acid/base disturbance. Such cases in birds may be associated with renal failure, unusual chloride intake patterns (drinking water, feed chloride sources, etc.), respiratory disease or high ambient CO2 concentrations on the farm, or high anion gap acidosis associated with starvation or low blood glucose, uremia (excess uric acid in blood), or lactic acid acidosis.

Practical Tips for Producers^*

• Regular Monitoring: Keep an eye on both total and ionized calcium levels to ensure your birds are healthy.
• Minimize Stress: Reduce stress during handling to maintain normal calcium levels.
• Balanced Diet: Ensure your birds get a balanced diet with the right amount of calcium and other nutrients.

*This information is educational and not a substitute for veterinary diagnosis or treatment

Conclusion

Understanding and monitoring calcium levels in your birds can help you maintain their health and productivity. By focusing on both total and ionized calcium, you can make informed decisions about their diet and management practices. Verax^TM provides valuable insights to understand and manage calcium levels in your animals, ensuring optimal health and performance.

Reference values are illustrative and may vary by analyzer, methodology age/strain and farm conditions.