4.2 Diagnostic Test and Procedure Selection

Quick Answer: Pick the first test that can change the plan, not the fanciest image. ECG before D-dimer in typical ACS. TSH before a thyroid panel. CBC before ferritin. Rapid strep, then culture in children if the rapid test is negative. Ultrasound before CT when ultrasound answers the question. Pregnancy test before ionizing pelvic imaging or a teratogenic workup. Do not test when pretest probability is negligible or when a positive result will only create an incidentaloma.

Selecting a test is an official Domain II skill on both the current Test Content Outline and the October 30, 2026 outline. The exam is not asking you to recite a laboratory catalog. It is asking whether you can match a presentation to a first-line study, know what would be confirmatory rather than initial, and recognize when the correct action is not to test.

Pretest probability is the first decision

Pretest probability is how likely the disease is before the result returns, based on the history, the exam, and the epidemiology of that patient. A 72-year-old smoker with sudden unilateral leg swelling has a high pretest probability of DVT. A 22-year-old runner with bilateral shin soreness after a new training block does not.

A test is useful only if it can move you across a decision threshold:

• If pretest probability is already very high, a negative imperfect test will not stop treatment (do not let a normal D-dimer cancel a high-probability PE).
• If pretest probability is very low, a positive imperfect test is more likely a false positive than a diagnosis (do not CT every young adult with a 20-minute tension headache).
• If the result cannot change management, do not order the test.

Write the clinical question on the order. Suspected pyelonephritis in pregnancy is not a fishing expedition for every conceivable pelvic finding.

Sensitivity and specificity without invented numbers

You do not need unpublished manufacturer percentages. You need the logic.

• Sensitivity is the proportion of people with the disease who test positive. A highly sensitive test, when negative, helps rule out disease (the SnNOut mnemonic). Rapid antigen tests and D-dimer are used this way in the right pretest lane.
• Specificity is the proportion of people without the disease who test negative. A highly specific test, when positive, helps rule in disease (SpPIn). A throat culture growing Streptococcus pyogenes is more specific than a sore throat.
• Predictive values ride on prevalence. The same D-dimer that is reassuring in a low-probability patient is uninterpretable in a high-probability patient.

Exam trap: quoting a memorized 94% and calling it official ANCC content. ANCC will not publish a kit insert. Reason from pretest lane + role of the test (rule out vs confirm), not from a number you invented in a study group.

When not to test

Low-value testing is a diagnosis skill, not a billing afterthought.

• Do not test to soothe the room when the pretest probability is negligible and the harm is real. A brain CT for a stereotyped migraine that matches the patient's 15-year history creates radiation and incidental findings without changing care.
• Incidentalomas are the downstream tax on unfocused imaging. An unnecessary abdominal CT finds an adrenal nodule, a lung nodule on the cutoff, and a 1.2 cm ovarian cyst — each of which then demands its own pathway. The FNP who ordered the first scan owns that cascade.
• Do not use a screening test as a diagnostic shortcut. A screening mammogram is the wrong next study for a palpable mass. A screening A1c is not how you evaluate a patient already in hyperglycemic crisis.
• Do not stack simultaneous overlapping tests when a sequence answers the question (TSH first, not TSH plus free T4 plus T3 plus antibodies on day one in an asymptomatic adult).

First-line versus confirmatory: the high-yield panel

Pharyngitis / strep. Estimate pretest (Centor/McIsaac conceptually: fever, no cough, tender anterior nodes, tonsillar exudate, age). If testing is indicated, start with a rapid antigen detection test (RADT). In children and adolescents, a negative RADT is followed by a throat culture (or a validated molecular test, depending on local protocol) because missing group A strep risks rheumatic sequelae. A positive RADT is specific enough to treat. In low-risk adults, backup culture after a negative RADT is often omitted because rheumatic fever is rare. Do not start with a neck CT for garden-variety pharyngitis.

UTI. In a nonpregnant, low-risk woman with classic cystitis, a urine dipstick (leukocyte esterase, nitrite) supports the clinical diagnosis and you may treat. Add a culture when the UTI is complicated, pregnant, recurrent, in a male, associated with pyelonephritis, or failing first-line therapy. Do not send every uncomplicated cystitis to culture, and do not treat asymptomatic bacteriuria except in pregnancy (and before selected urologic procedures).

Chest pain. The first test is a 12-lead ECG, obtained now, not after a D-dimer. Typical ACS is a coronary thrombosis problem; D-dimer is a PE rule-out tool for a different pretest question. Troponin belongs on an emergency pathway (next section). Do not send the patient to an outpatient coronary CT next week while they are still having rest pressure in your office.

Suspected PE. Use a structured pretest tool conceptually. PERC applies only when clinical suspicion is already low; if every PERC item is negative, you do not owe a D-dimer or a CT. If suspicion is not low, use Wells (or a similar validated score) to separate unlikely from likely. Unlikely → high-sensitivity D-dimer; a negative dimer stops the workup, a positive dimer proceeds to imaging. Likely → CT pulmonary angiography (or V/Q when CT contrast is contraindicated). Do not D-dimer a high-probability patient as a stall, and do not CT-PA a PERC-negative low-probability patient because oral contraceptives sound scary.

Hypothyroidism. The first test is TSH. Add free T4 when TSH is abnormal or when you suspect pituitary disease (low TSH would be the wrong first interpretation of central hypo). Thyroid ultrasound is for a nodule or asymmetric goiter, not for fatigue with a normal TSH. Thyroid antibodies confirm autoimmune mechanism after the function tests, not before.

Anemia. Start with a CBC (and a smear if the lab flags morphology). The MCV pattern then chooses the confirmatory tests: ferritin / iron studies for microcytic or suspected iron deficiency; B12 and folate for macrocytic anemia or neuropathy; mixed or unexplained normocytic anemia may need both plus reticulocyte count and a look at kidney and inflammation. Do not order a hemoglobin electrophoresis as the first test for a newly anemic adult with a ferritin of 6.

Chest x-ray versus CT. Chest x-ray is first for most primary-care pulmonary questions: suspected community pneumonia, congestive findings, large pneumothorax, and many cough-plus-fever pathways. CT is the next, not the first, test when you need a PE protocol, nodule characterization, cancer staging, or clarification of an abnormal or discordant film. Do not CT a previously healthy adult with a classic lobar pneumonia that you can treat and follow.

Ultrasound first — and pregnancy before radiation

Ultrasound is first-line when it answers the anatomic question without ionizing radiation:

• Right-upper-quadrant / biliary colic and cholecystitis (and often bile-duct dilation as a first look).
• DVT — duplex venous ultrasound, not a CT venogram, for a first suspected lower-extremity clot.
• Pelvis — suspected IUP or ectopic (with quantitative hCG), ovarian torsion concern, or most adnexal questions in clinic-to-ED handoff.
• Children — pyloric stenosis, intussusception, many appendicitis pathways, scrotal pain; ALARA is not optional in pediatrics.

Pregnancy test before teratogenic workup or ionizing imaging of the abdomen or pelvis. A late period, contraception use, or she says she cannot be pregnant is not a sensitive test. Obtain urine or serum hCG before CT of the abdomen/pelvis, before selected radiographs that beam the pelvis, and before you launch a workup that would include teratogenic drugs (isotretinoin planning, ACE inhibitor starts, methotrexate, and similar). Shielding does not replace a pregnancy test when the beam includes the uterus.

Cost and radiation are selection factors

The cheapest correct test is the one that answers the question once. A $20 TSH that diagnoses primary hypothyroidism beats a $2,000 endocrine panel that does not. Radiation is a selection variable, not only a consent paragraph: CT-PA, abdominal CT, and repeated films add up, especially in children, young adults, and pregnancy. Choose ultrasound or a clinical diagnosis when they are sufficient. Document why a higher-radiation study is the one that changes management.

Presentation → first test → why not the fancy test

Vignette

A 29-year-old woman on a combined oral contraceptive has 12 hours of sharp left-sided chest pain after a viral illness. She is not short of breath. Heart rate is 78, oxygen saturation 99%, no hemoptysis, no unilateral leg swelling, no prior VTE or surgery, and you judge her pretest probability of PE to be low. PERC is entirely negative.

The correct selection is no D-dimer and no CT-PA. Oral contraceptive use raises concern in the history, but it does not by itself move her into a likely-PE imaging lane. Ordering CT because it would be terrible to miss one is how incidental nodules and contrast reactions are born. If her pretest had been moderate and PERC inapplicable, the first test would have been a D-dimer, not a CT. If she had been hypoxic with a swollen leg and you judged PE likely, the first test would have been CT-PA — and she would not still be sitting in a primary-care hallway.