As an FNP, you'll order labs and interpret them every single day. The boards exam doesn't test memorization of normal ranges nearly as much as it tests interpretation — what does this number mean for this patient, and what do I do next?
Here's a primary-care–focused tour of the most-tested labs.
Every abnormal lab is a question, not an answer. The question is "what's the story?"
TSH and Thyroid Workup
Normal TSH 0.4–4.5 mIU/L (varies by lab).
- High TSH, low free T4: overt hypothyroidism. Start levothyroxine ~1.6 mcg/kg/day. Recheck TSH in 6–8 weeks.
- High TSH, normal free T4: subclinical hypothyroidism. Treat if TSH >10, pregnant, planning pregnancy, or symptomatic with positive antibodies.
- Low TSH, high free T4: overt hyperthyroidism. Workup: thyroid antibodies (TRAb for Graves), uptake scan. Treat: methimazole, beta blocker for symptoms, refer endocrine.
- Low TSH, normal free T4: subclinical hyperthyroidism. Watch unless persistent or symptomatic.
Pregnancy thyroid: TSH targets are trimester-specific. First trimester <2.5, second/third <3.0.
Hemoglobin A1c
- <5.7% — normal
- 5.7–6.4% — prediabetes
- ≥6.5% — diabetes (confirm on second test or with concurrent abnormal fasting glucose)
A1c reflects ~3 months of glucose. False results in: hemoglobinopathies, recent blood loss, pregnancy, anemia. In those cases, use fructosamine or fasting glucose instead.
LFTs
Pattern recognition is the goal.
- AST > ALT (ratio >2:1): Alcoholic liver disease.
- ALT > AST: Non-alcoholic fatty liver, viral hepatitis, drug-induced.
- Markedly elevated AST/ALT (>1000): Acute viral hepatitis, ischemia, drug toxicity (acetaminophen).
- Elevated alkaline phosphatase + GGT: Cholestasis (gallstones, biliary obstruction, primary biliary cholangitis).
- Elevated alkaline phosphatase + normal GGT: Bone source (Paget's, fracture, growth).
- Elevated bilirubin: Indirect (hemolysis, Gilbert's) vs. direct (hepatocellular, cholestatic).
CBC Patterns
Anemias by MCV
- Microcytic (MCV <80): iron deficiency (most common), thalassemia, chronic disease, lead.
- Normocytic (MCV 80–100): chronic disease, acute blood loss, early iron deficiency, hemolysis.
- Macrocytic (MCV >100): B12 or folate deficiency, alcohol, hypothyroidism, drugs (methotrexate, hydroxyurea).
Iron studies
- Iron deficiency: low ferritin, low serum iron, high TIBC, low transferrin saturation.
- Anemia of chronic disease: normal/high ferritin, low iron, low TIBC.
WBC patterns
- Neutrophilia: bacterial infection, stress, steroids.
- Lymphocytosis: viral infection.
- Eosinophilia: parasites, allergies, asthma, drug reaction.
- Neutropenia: viral, chemotherapy, medications.
Test Pearl: An MCV pattern is your fastest first cut for anemia. Build the differential from there.
BMP / CMP
- BUN/Cr ratio: >20:1 suggests dehydration or GI bleed. ~10:1 with both elevated suggests intrinsic renal disease.
- Hyponatremia: SIADH, heart failure, cirrhosis, hypothyroidism, adrenal insufficiency, diuretics.
- Hypernatremia: dehydration, diabetes insipidus.
- Hyperkalemia: renal failure, ACE/ARB/spironolactone, hemolysis (in vitro), Addison's. Get an EKG.
- Hypokalemia: diuretics, GI losses, hyperaldosteronism. Causes EKG changes (U waves, T wave flattening).
- eGFR: <60 for 3+ months defines CKD. Stage based on eGFR.
Lipid Panel
- Goal LDL based on ASCVD risk (covered in another post).
- Triglycerides >500 → pancreatitis risk; treat aggressively (fibrates, omega-3s).
- Non-HDL cholesterol (total cholesterol − HDL) often more clinically useful than LDL alone.
Urinalysis
- Nitrites + leukocyte esterase: bacterial UTI.
- Glucose: diabetes (or rarely renal glycosuria).
- Protein: kidney disease, preeclampsia, exercise-induced.
- Blood: infection, stones, cancer, glomerular disease, menstruation contamination.
- Ketones: DKA, starvation, low-carb diet.
- Casts: RBC casts = glomerulonephritis; WBC casts = pyelonephritis; muddy brown casts = ATN.
B12 and Folate
- B12 deficiency: macrocytic anemia + neurologic symptoms (paresthesias, ataxia). Check methylmalonic acid and homocysteine if borderline.
- Folate deficiency: macrocytic anemia without neurologic findings. Always check B12 before treating folate alone — folate replacement masks B12 deficiency anemia while neuro damage progresses.
Vitamin D
- Deficiency: 25-OH vitamin D <20 ng/mL.
- Insufficiency: 20–30 ng/mL.
- Sufficient: >30 ng/mL.
- Replacement: 50,000 IU weekly x 8 weeks for deficiency, then 1500–2000 IU daily maintenance.
How the Exam Tests Labs
Don't memorize ranges. Memorize patterns and stories. The exam describes a patient and asks what next. You answer based on what the lab pattern is telling you about the underlying process.