1. Chapter 4. Proteins

Blood plasma composition

About 93% water and 7% mainly proteins.

Functions of plasma proteins

Enzymes, antibodies, coagulation factors, transport, nutrition, oncotic pressure, and acid-base balance.

Albumin main function

Maintains oncotic pressure.

What happens when albumin is very low?

Water leaves vessels and causes edema.

Serum vs plasma

Serum lacks fibrinogen; plasma contains fibrinogen.

Basic protein status

Total proteins, albumin, and globulins.

Globulin calculation

Globulins = total proteins − albumin.

Total protein method

Biuret method.

Albumin method

Bromocresol green method.

Hyperproteinemia

Increased plasma protein concentration.

Hyperproteinemia in dehydration

Relative increase due to reduced water volume.

Marked hyperproteinemia

Typical of multiple myeloma due to monoclonal immunoglobulins.

Hypoproteinemia

Decreased plasma protein concentration.

Causes of hypoproteinemia

Nephrotic syndrome, burns, wounds, hemorrhage, ulcerative colitis, malabsorption, and malnutrition.

Immunoturbidimetry and immunonephelometry

Methods for measuring specific proteins using antigen-antibody reactions.

Antigen-antibody reaction

Specific antigen and antibody form immune complexes.

Antibody excess zone

Correct measuring zone for immunochemical methods.

Equivalence zone

Zone with maximal precipitation.

Antigen excess zone

Too much antigen forms small soluble complexes and can give misleading results.

Turbidimetry

Measures transmitted light through a cloudy solution.

Nephelometry

Measures scattered light from immune complexes.

Main difference between turbidimetry and nephelometry

The reaction is the same, but detection of light is different.

MININEPHplus

Semi-automated laser nephelometer using 670 nm laser light.

Latex-enhanced immunoturbidimetry

Uses latex particles to amplify immune complex formation and increase sensitivity.

CRP

C-reactive protein, an acute-phase protein made in the liver.

CRP normal value

Up to 10 mg/L.

CRP rises in

Inflammation, infection, trauma, and tissue necrosis.

CRP 10–40 mg/L

Mild inflammation or viral infection.

CRP 40–200 mg/L

Acute inflammation or bacterial infection.

hsCRP

High-sensitivity CRP used for cardiovascular risk assessment.

Rheumatoid factor

Autoantibodies against own immunoglobulins.

High RF in rheumatoid arthritis

Indicates poorer prognosis.

Transferrin

Iron transport protein.

Transferrin increases in

Low plasma iron, pregnancy, and estrogen therapy.

Transferrin decreases in

Chronic infection, malignancy, liver disease, and kidney disease.

Haptoglobin

Binds free hemoglobin in plasma.

Haptoglobin decreases in

Hemolytic anemia and liver disease.

β2-microglobulin

Low-molecular-weight protein filtered by glomeruli and reabsorbed in proximal tubules.

High urinary β2-microglobulin

Marker of tubular proteinuria.

CRP latex agglutination

Qualitative or semiquantitative method where visible agglutination means positive CRP.

Fibrinogen

Plasma protein precursor of fibrin and acute-phase protein.

Fibrinogen increases in

Inflammation, infection, trauma, and pregnancy.

Fibrinogen decreases in

DIC, systemic fibrinolysis, and severe liver dysfunction.

Clauss method

Method measuring fibrinogen activity by clotting time after excess thrombin.

Clauss method relation

Fibrinogen concentration is inversely proportional to clotting time.

Sample for fibrinogen

Citrated plasma.

Proteinuria

Proteins in urine above reference values.

Urine protein reference at rest

50–80 mg/24 h, often <100 mg/24 h used.

Urine protein after physical exertion

Can increase up to 300 mg/24 h.

Glomerular proteinuria

Increased glomerular permeability, mainly albuminuria.

Tubular proteinuria

Impaired proximal tubular reabsorption with low-molecular-weight proteins in urine.

Overload proteinuria

Proteinuria due to high plasma concentration of small proteins.

Examples of overload proteinuria

Hemoglobinuria, myoglobinuria, and Bence-Jones proteinuria.

Postrenal proteinuria

Proteinuria due to urinary tract inflammation or carcinoma.

Urine albumin test strip

Albumin detection by protein error of indicator; semiquantitative.

False positive urine protein strip

Alkaline urine pH >9 or contamination with detergents/disinfectants.

Sulfosalicylic acid test

Sensitive qualitative protein test causing turbidity.

Proteins detected by sulfosalicylic acid

Albumin, globulins, and Bence-Jones protein.

Pyrogallol red method

Quantitative photometric method for urine or CSF proteins measured at 600 nm.