Albumin

Serum (clotted blood)

Increases with dehydration
Decreases usually due to enteric losses

An hepatic-synthesised, negatively charged, mildly acidic protein which forms the major plasma oncotic colloid as well as binding cationic electrolytes (especially Ca2+ and Mg2+), H+ and some other metabolites and drugs.

High concentrations reflect dehydration.

Low concentrations usually reflect enteric losses although sometimes might follow renal injury, chronic inflammation, haemorrhage, loss into effusions or longstanding severe hepatic insufficiency.

Profoundly low concentrations (<20-25 g/L) are generally from enteric diseases including parasitism, Lawsonia intracellularis, NSAID toxicity or chronic infiltrative bowel diseases.

Whether or not oedema develops with hypoalbuminaemia depends on several factors including how quickly albumin has fallen as well as whether or not the globulins are also lost, or have even increased due to inflammation. Plasma colloidal oncotic pressure can be measured with an osmometer or can be estimated from the formula:

Colloidal oncotic pressure (mmHg) ≈ 0.55 x [Albumin g/L] + 0.25 x [Globulin g/L] – 4.4

Normal adult horses tend to have values > 25 mmHg. Oedema generally forms with acute decreases to <15 mmHg although gradual protein losses may be tolerated to as low as 10-12 mmHg.

Albumin is mildly acidic and therefore hypoalbuminaemia has a mild alkalinising effect, although this is often counteracted by coexisting acidifying issues such as hyperlactaemia and hyponatraemia.