4.3 Interpreting Labs and Imaging

Quick Answer: Read patterns, not a photocopied reference range. Confirm diabetes with ADA cutoffs (A1c ≥6.5%, fasting glucose ≥126 mg/dL, 2-hour OGTT ≥200 mg/dL, or random ≥200 mg/dL plus symptoms) unless the patient is already in crisis. Let the CBC MCV choose ferritin versus B12. Let AST/ALT versus ALP sort hepatocellular from cholestatic injury. Troponin is an emergency test, not a hallway send-out. A normal creatinine in a frail elder or a normal nonpregnant TSH in the first trimester can still be the wrong number for that physiology.

Interpreting diagnostic tests and procedures is already embedded in Domain II items. Beginning October 30, 2026, the Test Content Outline names it as an explicit skill. The exam will not hand you a laboratory's private reference interval and ask you to recite it. It will hand you a pattern and ask what you do next.

Do not invent numeric reference ranges that vary by analyzer, sex, or lab. Teach guideline action thresholds (ADA diabetes cutoffs, ACC/AHA lipid treatment triggers) and directional patterns (high TSH / low free T4, cholestatic versus hepatocellular enzymes).

CBC patterns

Start with hemoglobin, MCV, RDW, leukocyte count and differential, and platelets. Then name the pattern.

• Microcytic anemia (low MCV): iron deficiency, thalassemia trait, and some anemia of inflammation. Next test is ferritin (and iron studies). A frankly low ferritin is iron deficiency. Thalassemia trait usually has a low MCV out of proportion to a mild anemia and a normal or high RDW pattern that is less iron-like — consider electrophoresis after iron deficiency is excluded.
• Normocytic anemia: acute blood loss, anemia of inflammation, chronic kidney disease, mixed iron-plus-B12 deficiency, or early any-cause anemia. Check reticulocyte response, creatinine, and inflammation before you call it unexplained.
• Macrocytic anemia (high MCV): B12 deficiency, folate deficiency, alcohol, certain drugs (including some antiretrovirals and anticonvulsants), and marrow disorders. B12 and folate are the first add-on labs; neuropathy tilts the mechanism toward B12 even if the anemia is mild.
• Leukocytosis with a left shift supports a bacterial stress response but is not a culture. Isolated eosinophilia points you toward allergy, parasite, or adrenal insufficiency, not community pneumonia.
• Isolated thrombocytopenia is medicines, immune thrombocytopenia, liver disease, or pregnancy-related syndromes — not a reflex bone-marrow biopsy on visit one.
• Pancytopenia is marrow failure, infiltration, or hypersplenism until proven otherwise. That pattern is refer, not a 3-month observation.

A rising hemoglobin on iron is expected; a falling hemoglobin after you started iron means you missed the bleed or the diagnosis.

CMP: sodium, potassium, kidney, and liver patterns

Sodium. Interpret hyponatremia with volume status and serum osmolality conceptually: hypovolemic (losses, diuretics), euvolemic (SIADH, hypothyroidism, adrenal insufficiency), or hypervolemic (heart failure, cirrhosis, nephrosis). Symptomatic or severe hyponatremia is an emergency, not a primary-care salt-tablet experiment. Hypernatremia is usually water deficit — think access to water in the frail.

Potassium. Hyperkalemia with ECG changes or a marked elevation is emergency (membrane-stabilization and shifting happen in acute care). Look at the specimen: hemolysis falsely raises potassium. Hypokalemia with loop or thiazide diuretics, vomiting, or diarrhea is common in clinic; replace and find the driver, and remember that low magnesium keeps potassium low.

Creatinine and eGFR. A rising creatinine is acute kidney injury until you have a reason it is not (obstruction, medicines, dehydration, glomerulonephritis). eGFR estimates chronic filtration; it is less reliable in extremes of muscle mass. A creatinine that prints normal in a 45-kg frail 88-year-old can still hide a reduced GFR — that is not a license to dose metformin or an NSAID as if the kidney were 40 years old. Persistent eGFR below 60 mL/min/1.73 m² is the CKD threshold used in guideline language; confirm chronicity.

AST/ALT versus ALP and bilirubin.

• Hepatocellular pattern: AST and ALT dominate (viral hepatitis, NAFLD/NASH, ischemic liver, many drugs).
• Cholestatic pattern: ALP dominates, supported by GGT or conjugated bilirubin (bile-duct obstruction, infiltrative disease, some drugs).
• AST:ALT ratio greater than 2 conceptually supports alcohol when the story matches; it is not a diagnosis by itself.
• Isolated unconjugated bilirubin in a well adult is often Gilbert syndrome or hemolysis; conjugated (direct) bilirubin is never Gilbert.

Do not invent an upper-limit-of-normal that your hospital card happens to use. Describe the pattern and the action (stop the hepatotoxin, image the ducts, refer for jaundice or synthetic dysfunction).

ADA diabetes diagnostic cutoffs

Use the American Diabetes Association thresholds. They are guideline action numbers, not lab-specific ranges.

Confirm an abnormal result with a second abnormal test — either a different test from the same sample or a repeat on a later day — unless there is unequivocal hyperglycemia or a hyperglycemic crisis. A single A1c of 6.6% in an asymptomatic adult is not a completed diagnosis. Prediabetes bands (A1c 5.7–6.4%, fasting 100–125 mg/dL, 2-hour OGTT 140–199 mg/dL) change counseling intensity; they are not diabetes.

A1c and glucose can disagree. When they do, repeat the discordant test and believe the pathophysiology (hemolysis, transfusion, and pregnancy make A1c less trustworthy).

Lipids, thyroid, urine, and natriuretic peptides

Lipids. Fasting is not required for a first lipid panel in many adults; repeat fasting if triglycerides are markedly elevated. Do not recite a lab's optional desirable LDL. Act on guideline triggers: LDL-C ≥190 mg/dL is a high-intensity statin indication in adults; triglycerides ≥500 mg/dL raise pancreatitis risk and need specific management, not only a diet handout. Familial patterns (very high LDL plus family premature CHD or tendon xanthomas) change the referral, not only the prescription.

TSH and free T4.

In pregnancy, use a pregnancy-specific or trimester-specific TSH interval. A TSH that looks normal on a nonpregnant report can be too high in the first trimester. That is interpretation, not a new lab.

Urinalysis and culture. Leukocyte esterase, nitrite, white cells, and bacteria support infection when the story matches. Many epithelial cells mean contamination — repeat, do not treat the cup. Protein plus blood plus casts is a glomerular clue, not a simple cystitis. Culture interprets colony count in clinical context; a clean-catch contaminant is not pyelonephritis. Pregnant bacteriuria is treated even when the patient feels well.

BNP / NT-proBNP. An elevated natriuretic peptide supports heart failure when dyspnea is undifferentiated. A very low value makes decompensated HF less likely. Obesity can lower measured BNP; kidney disease can raise it. The peptide does not replace the exam, the chest x-ray, or the echo, and it is not a reason to keep a hypoxic patient in clinic awaiting a send-out.

Troponin is not a primary-care waiting-room test

Troponin is a marker of myocardial injury. In the right story (rest pressure, diaphoresis, ischemic ECG change, equivalent dyspnea in a high-risk adult), a troponin belongs on an emergency pathway with serial ECGs and a team that can manage ACS. Drawing a troponin in the office and sending the patient to the grocery store to await a callback is a systems failure, not thoughtful interpretation. A rising troponin after a demand insult (sepsis, tachyarrhythmia) is still injury and still not a clinic observation plan. Do not delay transfer to wait for the first clinic value.

Chest x-ray patterns the FNP must act on

You are not a radiologist. You are the clinician who cannot file these films under routine:

• Infiltrate / consolidation in a febrile, tachypneic patient is pneumonia until you have a better explanation. Assess severity and decide office treatment versus transfer. A normal film does not fully exclude early pneumonia if pretest is high — treat the patient, not only the jpeg.
• Pneumothorax (visceral pleural line, absent peripheral markings, possible mediastinal shift) is emergency evaluation, not a 1-week repeat film and an incentive spirometer.
• Widened mediastinum after chest pain or a tearing back pain is an acute aortic syndrome until a definitive vascular study says otherwise. That patient leaves by emergency transport, not with a primary-care CT requisition in hand.
• Pulmonary edema pattern (vascular redistribution, Kerley lines, effusion, bat-wing alveolar edema) corroborates the congestion mechanism from section 4.1.
• A new nodule is a tracking and risk-stratification problem (Fleischner conceptually), not a same-day cancer diagnosis and not something to ignore.

When normal is abnormal for age or state

• eGFR / creatinine in the frail: low muscle mass keeps creatinine deceptively modest. Dose kidneys, not birthday candles. Avoid NSAIDs and other nephrotoxins you would never give an overt CKD patient.
• TSH in pregnancy: nonpregnant intervals miss first-trimester hypothyroidism. Use a pregnancy interval or a pregnancy-aware endocrinology pathway.
• Heart rate and blood pressure need age context (next section). An adult-normal heart rate of 70 can be a preterminal infant. An adult-normal creatinine of 1.0 mg/dL can be AKI in a pregnant patient whose GFR should have risen.
• Temperature in older adults: absence of fever does not exclude infection. A falling mental status plus a normal temperature can still be sepsis or pneumonia.

Vignette

A 54-year-old woman is asymptomatic at a new-patient visit. A1c is 6.7%. Fasting glucose drawn the same day is 118 mg/dL. TSH is 2.1 mIU/L on a nonpregnant assay; she is not pregnant. CBC shows hemoglobin 10.8 g/dL and a low MCV; ferritin is pending.

Interpretation, not a reflex label: the A1c meets the ADA diabetes threshold, but the fasting glucose does not, and she has no crisis — confirm with a repeat diagnostic test before you name diabetes. The TSH is unremarkable in a nonpregnant adult. The microcytic anemia is a pattern that waits on ferritin; if ferritin is low, the implication is iron deficiency and a bleed evaluation, not a diagnosis of anemia of chronic disease. None of these numbers is a license to start insulin, ignore the anemia, or order a thyroid ultrasound.