Biochemical tests in pregnancy

Editor, – In addition to the tests mentioned in the article 'Abnormal laboratory results: Biochemical tests in pregnancy' (Aust Prescr 2005;28:98-101), there are several other tests where the changes in the normal ranges during pregnancy are of clinical importance.

• Serum bicarbonate falls by approximately 4 mmol/L to compensate for the respiratory alkalosis which results from elevated progesterone concentrations stimulating respiratory drive.¹
• Serum vitamin B12falls in 25% of pregnant women such that a value of greater than 100 pmol/L should be regarded as normal for pregnancy. In the absence of folate deficiency serum homocysteine is of value in establishing true B12deficiency in pregnancy.^2,3
• Erythrocyte sedimentation rate rises significantly (often up to 100 mm/hour).⁴
• White cell count rises due to neutrophil leucocytosis.⁵
• D-dimer becomes elevated in second and third trimesters.⁶
• Free protein S concentrations fall significantly.⁷
• Creatine kinase (MB subfraction) rises after vaginal delivery.⁸
• Serum troponin may be elevated in pre-eclampsia making diagnosis of myocardial ischaemia problematic if mothers develop pulmonary oedema.⁹
• Plasma renin activity and serum aldosterone rise masking detection of primary aldosteronism as a cause of pre-gestational hypertension in pregnancy.¹⁰

Adam Morton
Senior staff specialist, Endocrinology and Obstetric Medicine
Mater Adult Hospital
Brisbane

Dr H.A. Tran, author of the article, comments:

Dr Morton's comments on other laboratory parameters that change during pregnancy are very much appreciated. The article aimed to highlight biochemical changes in common tests without being overly exhaustive. Generally speaking, pregnancy is a volume retentive, prothrombotic and nutritionally challenged state which results in all the corresponding changes described.

The hypervolaemic state is the result of an activated renin-angiotensin system with markedly elevated aldosterone concentrations and plasma renin activity. The normal physiological control of this system however remains intact, distinguishing it from primary hyperaldosteronism during pregnancy.¹¹

The prothrombotic state is highlighted by the elevated d-dimer concentrations and reduced free protein S concentrations. The latter is the result of elevated protein binding capacity which is typical of pregnancy. Similarly, elevated transcobalamin and haptocorrin concentrations contribute to the reduction in cobalamin concentrations¹²although preferential fetal transfer during pregnancy also adds to the problem, particularly in vegans. It is probably more cost-effective to replenish B12storage empirically for the duration rather than relying on homocysteine concentrations to diagnose B12deficiency. Erythrocyte sedimentation rate, by way of physiological anaemia during pregnancy, is expected to be elevated but usually not to 100 mm/hour. The mean peak ranges from 50-70 mm/hour depending on the gestational age.¹³Thus, where it exceeds 100 mm/hour it is important that active inflammation or infection is excluded. Similarly, while white cell count can rise up to 15-16 x 106/mL, the majority often do not exceed the non-pregnant reference range.¹⁴