Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (271 page)

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PREGNANCY AND OBSTETRIC MONITORING OF THE FETUS AND PLACENTA

PREGNANCY

   Normal Laboratory Values Altered by Pregnancy
*

   
Hematology
: RBC mass increases 20%, but plasma volume increases approximately 40% causing RBC, Hb, and Hct to decrease approximately 15%. WBC increases 66%. Platelet count decreased by average 20%. ESR increases markedly during pregnancy, making this a useless diagnostic test during pregnancy. Occasionally cold agglutinins may be positive and osmotic fragility increased.

   
Renal function tests
: Respiratory alkalosis with renal compensation. Normal − pCO
₂
= approximately 30 mEq/L, normal HCO
₃
^-
= 19–20 mEq/L. Serum osmolality decreases 10 mOsm/kg during first trimester. Increased GFR 30–50% early until approximately 20 weeks postpartum. Renal plasma flow increases 25–50% by midpregnancy. BUN and creatinine decrease 25%, especially during first half of pregnancy. BUN of 18 mg/dL and creatinine of 1.2 mg/dL are definitely increased (abnormal) in pregnancy, although normal in nonpregnant women.
Beware of BUN
>
13 mg/dL and creatinine
>
0.8 mg/dL
. Serum uric acid decreases 35% in first trimester (normal = 2.8–3.0 mg/dL); returns to normal by term. Serum aldosterone, angiotensins I and II, and renin are increased although secondary hyperaldosteronism may also be seen with toxemia of pregnancy.

   
Urinalysis
: Urine volume is not increased. Glycosuria occurs in >50% of patients due to impaired tubular resorption. Lactosuria should not be confused with glucose in urine. Proteinuria (200–300 mg/24 hour) is common (approximately 20% of patients); worsens with underlying glomerular disease. Urine porphyrins may be increased. Urinary gonadotropins (human chorionic gonadotropin, hCG) are increased. Urine estrogens increase from 6 months to term (≤100 μg/24 hours). Urine 17-ketosteroids rise to upper limit of normal at term.

   
Serum protein findings
: Serum total protein decreases 1 g/dL during first trimester; remains at that level. Serum albumin decreases 0.5 g/dL during first trimester and 0.75 g/dL by term.

   Serum α-1 globulin increases 0.1 g/dL. Serum α-2 globulin increases 0.1 g/ dL. Serum β-globulin increases 0.3 g/dL.

   
Chemistry
: Fasting blood glucose decreases 5–10 mg/dL by end of first trimester. Serum calcium decreases 10%. Serum magnesium decreases 10%. No changes are found in serum levels of sodium (normal = approximately 135 mEq/L), potassium, chloride, or phosphorus. Serum T
₃
uptake is decreased and T
₄
is increased. T
₇
(T
₃
× T
₄
) is normal. TBG is increased. (Check tests for thyroid function.) Serum progesterone is increased.

   
Enzyme studies
: No changes are found in serum levels of amylase, AST, ALT, LD, ICDH, acid phosphatase, and α-hydroxybutyrate dehydrogenase. Serum CK decreases 15% by 20 weeks of gestation; increases at beginning of labor to peak 24 hours postpartum; and then gradually returns to normal. CK-MB is detected at onset of labor in approximately 75% of patients with peak 24 hours postpartum and then returns to normal. Serum LD and AST levels remain low. Serum ALP increases (200–300%) progressively during the last trimester of normal pregnancy caused by an increase of heat-stable isoenzyme from the placenta. Serum LAP may be increased moderately throughout pregnancy. Serum lipase decreases 50%. Serum pseudocholinesterase decreases 30%.

   
Lipid studies
: Serum phospholipid increases 40–60%. Serum triglycerides increase 100–200%. Serum cholesterol increases 30–50%.