Raised alkaline phosphatase in pregnancy

Alkaline phosphatase (AP) is known to be produced by the liver, bones, small intestine, and kidneys, and different AP isoforms are also expressed by the placenta during pregnancy

• average amount of AP in one human term placenta amounts to 40 mg
  
  
• placental isoforms are known as heat stable AP, because they are heat resistant at 60°C, a property that is the main criterion for distinguishing them from the other isoenzymes
  
  
• in early pregnancy, the tissue non-specific AP isoenzyme is mainly expressed in the placenta, and reaches a peak value around 10 weeks of pregnancy
  
  
• at the end of the second trimester, most of the AP activity comprises term placental AP isoenzymes (90% of which are the P1 type, 10% the P2 type) produced by the syncytiotrophoblasts, and these isoenzymes appear in maternal serum between the 15th and 26th weeks of pregnancy
  • their plasma concentrations increase exponentially during gestation - they are present at concentrations three times greater than those seen in non-pregnant women - and have a long half life (seven days) postpartum
    
    
  • extremely high AP concentrations should arouse a suspicion of bone, hepatic, endocrine, and renal diseases, malignancy, and drug treatment, but can also be associated with heavy smoking and pregnancy (1)
    
    
  • in a pregnant patient, elevation of ALP may be related to HELLP syndrome and intrahepatic cholestasis (2)
    
    
  • AP electrophoresis can be useful to distinguish placental from hepatic or bone isozymes
    
    
  • usually, AP production or diffusion in maternal serum is not major and total serum ALP level remains normal (2)
    • some cases of unusual elevation of extremely high levels placental AP have been described (3,4)

Reference:

• 1) Boronkai A et al. Extremely high maternal alkaline phosphatase serum concentration with syncytiotrophoblastic origin. J Clin Pathol. 2005 Jan;58(1):72-6
• 2) Delluc C et al. Elevation of alkaline phosphatase in a pregnant patient with antiphospholipid syndrome: HELLP syndrome or not? Rheumatology (Oxford). 2008 Apr;47(4):554-5. Epub 2008 Feb 27.
• 3) Vongthavaravat V et al.Isolated elevation of serum alkaline phosphatase level in an uncomplicated pregnancy: a case report. Am J Obstet Gynecol. 2000 Aug;183(2):505-6.
• 4) Wojcicka-Bentyn J et al.Extremely elevated activity of serum alkaline phosphatase in gestational diabetes: a case report.Am J Obstet Gynecol. 2004 Feb;190(2):566-7