Plasma proteins are measured by refractometry, as part of the CBC. Certain substances, such as urea, glucose, cholesterol, lipoproteins, and excess anticoagulant, may interfere with the refractometric protein reading, falsely elevating the result. The difference between plasma and serum protein is sometimes greater than can be accounted for by the lack of fibrinogen in serum and is not explained by any known interfering substances. Presumably there are unknown molecules that may also affect light refraction in the plasma.

Total serum protein and albumin are included in many biochemical panels and are measured spectrophotometrically. Globulins are not measured directly, but are calculated by subtracting the albumin concentration from the total serum protein concentration.

Total protein to fibrinogen ratio

Fibrinogen determination, using the heat precipitation method, is a component of the CBC in large animals. Fibrinogen is a useful indicator of inflammation because it is a positive acute phase protein (APP). Although fibrinogen is also increased in small animals with inflammatory disease, the levels are not sufficiently high to be detected using the heat precipitation method, and fibrinogen is more likely to be measured when investigating coagulation problems rather than inflammatory conditions (see Chapter 4: Hemostasis).

In cattle, hyperfibrinogenemia is sometimes the only CBC finding indicating inflammation. Many laboratories report the plasma total protein to fibrinogen ratio in order to help determine if the hyperfibrinogenemia is relative or absolute (i.e. due to dehydration or secondary to inflammation). With dehydration, total protein and fibrinogen will increase to the same degree, while with inflammation there will be a disproportionate increase in fibrinogen, resulting in a low ratio. A ratio <10 in cattle or <15 in horses is most consistent with true hyperfibrinogenemia (see Table 1). Values in the grey zone are not useful in deciding if inflammation is present.

Other positive acute phase proteins

Besides fibrinogen, there are several other positive APPs that are occasionally measured, including serum C-reactive protein, serum amyloid A, haptoglobin, and ferritin. These positive APPs increase with inflammation, but can also be increased with other conditions, for example, haptoglobin can increase in dogs receiving glucocorticoid therapy. Pregnancy and lactation will increase C-reactive protein in dogs and cattle, respectively.

Serum protein electrophoresis

Serum proteins can be fractionated electrophoretically based on size and electrical charge. The separated bands are stained and scanned with a densitometer, which generates an electrophoretogram tracing and quantitates each fraction (see Case 2: Zed). In addition to an albumin peak, α1, α2, β1, β2, and γ globulin peaks may be seen. Due to species variation in whether the α and β fractions form two peaks each or one, the total number of peaks is between 3 and 5. The α globulins include, but are not limited to, acute phase proteins, α-lipoproteins, thyroxine binding proteins, and antithrombin. The β globulins include, but are not limited to, β-lipoprotein, transferrin, complement components, plasminogen, and fibrinogen (in plasma, not serum). The γ globulins contain all the immunoglobulins; however, IgM and IgA occasionally migrate into the β range.