Colloid Osmotic Pressure (COP)

Colloid osmotic pressure (COP) describes the ability of the intravascular space to retain fluid. The osmotic pressure gradient to retain fluid is primarily due to plasma protein concentration. Multiplying serum albumin concentration (g/dL) times six can approximate COP values. Loss of plasma proteins or increases in intravascular fluid causes COP to decrease.

Measurement of COP is useful in diagnosis of pregnancy-induced hypertension, hypovolemic shock, and assessing the risk of pulmonary edema during tocolytic therapy. In a normal pregnancy, maternal COP values decrease throughout gestation, reaching a nadir of 22.4 +/- 0.5 mm Hg between 34 and 36 weeks. This trend closely parallels the decline in maternal serum albumin concentration. Patients at term with pregnancy induced hypertension have lower COP values, due to proteinuria and decreased serum protein levels. Postpartum, both normotensive and hypertensive patients have COP values that are markedly lower than they were during pregnancy. Several explanations have been given for this decrease including; supine positioning during labor and delivery, blood loss during delivery, and administration of crystalloid fluids during labor.

Reference range during pregnancy is: 

COP values fall during tocolytic therapy, with the lowest values occurring nine hours after initiation of therapy. Long term therapy reduces COP more dramatically, due to vasodilatation and an increase in plasma volume. Patients with low or decreasing COP values are at greater risk of developing pulmonary edema.  

Specimen requirement is one green top (sodium heparin) tube of blood. Hemolysis and prolonged tourniquet time can artificially increase COP values.

References

Morissette MP. Colloid osmotic pressure: its measurement and clinical value. Can Med Assoc J. 1977 Apr 23;116(8):897-900.

Wu PY, Udani V, Chan L, Miller FC, Henneman CE. Colloid osmotic pressure: variations in normal pregnancy. J Perinat Med. 1983;11(4):193-9.