Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (926 page)

   End-stage renal disease; values ≥1,000 μg/L are not uncommon. Values <200 μg/L are specific for iron deficiency in these patients.

Decreased In

   Iron deficiency

   Hemodialysis

   Limitations

   In hepatic, malignant, and inflammatory conditions, ferritin levels can be normal. In such cases, bone marrow stain of iron may be used to exclude iron deficiency.

   Transferrin saturation is more sensitive to detect early iron overload in hemochromatosis; serum ferritin is used to confirm diagnosis and as an indication to proceed with liver biopsy. Ratio of serum ferritin (in ng/mL) to ALT (in IU/L) >10 in iron-overloaded thalassemic patients but averages ≤2 in viral hepatitis; ratio decreases with successful iron chelation therapy.

   Increases with age, is higher in men than in women, in women who use oral contraceptives, and in persons who eat red meat compared to vegetarians.

FETAL BIOPSY

See Prenatal Screening.

FETAL BLOOD SAMPLING (PERCUTANEOUS UMBILICAL BLOOD SAMPLING [PUB], CORDOCENTESIS)

See Prenatal Screening.

FETAL LUNG MATURITY (FLM)—LAMELLAR BODY COUNTS (LBC)

   Definition

   Most common test currently use in the United States. LBCs are concentrically layered “packages” of phospholipids produced by type II pneumocytes. LBCs represent the storage form of surfactant. They are present in amniotic fluid in increasing quantities as gestation advances. They are similar in size as platelets and can be counted in most hematology analyzers.

   
Normal range:

   Mature fetal lungs: ≥50,000/μL

   Borderline: 15,000–50,000/μL

   Immature fetal lungs: ≤15,000/μL