Whats and Hows- Diagnosis

Confirm, stage, and characterize disease

Diagnostic tests are specific and confirmatory. Use them to verify suspected conditions from screening and to plan treatment/consults.

Order with a clinical question in mind and a plan for the result.

Imaging — Core Modalities

confirmation • localization • complications

X-ray (Radiography)

Best for: Fracture/dislocation, chest lines/tubes, bowel obstruction pattern, osteoarthritis.

Limits: Early stress fractures, subtle intra-articular injuries, soft tissue detail.

CT (Computed Tomography)

Best for: Intracranial bleed, complex fractures, visceral injury, PE (CTPA), appendicitis/obstruction.

Limits: Radiation; less sensitive than MRI for marrow/ligament/cord pathology.

MRI (Magnetic Resonance)

Best for: Ischemic stroke (DWI), ligament/meniscus/tendon, spinal cord/cauda equina, osteomyelitis.

Limits: Time, availability, metal/device contraindications; less ideal for acute hemorrhage than CT.

Which Modality for Which Pathology?

quick reference

Clinical Question	First-Line	Why	If Negative / Add-On
Head trauma, sudden severe headache	CT head (non-contrast)	Fast, detects hemorrhage, mass effect	MRI brain for subtle lesions; LP for SAH if CT negative and high suspicion
Focal neuro deficit < 24h	CT head (rule out bleed)	Exclude ICH before thrombolysis	MRI DWI for ischemia; CTA/MRA for vessel occlusion
Chest pain with PE suspicion	CT Pulmonary Angio (CTPA)	Direct clot visualization	V/Q scan if CT contraindicated
Suspected pneumonia, effusion	CXR	Quick overview of lungs/pleura	CT chest for complications/occult disease; US for effusion guidance
Appendicitis, obstruction, perforation	CT Abd/Pelvis (± contrast)	High accuracy; shows complications	US first in pregnancy/peds; MRI if CT contraindicated
RUQ pain, biliary disease	RUQ Ultrasound	Stones, wall thickening, CBD size	HIDA for cystic duct obstruction; MRCP for ducts
Fracture/dislocation suspected	X-ray	Initial bony assessment	CT for complex/occult; MRI for stress fracture or osteonecrosis
Osteomyelitis/septic joint	X-ray (baseline)	Periosteal change/effusion (late)	MRI for marrow edema/early OM; US for effusion tap
Cauda equina / cord compression	MRI spine (urgent)	Cord, roots, discs, abscess	CT myelogram if MRI not possible
AAA suspected	Bedside US	Rapid diameter screen	CTA for operative planning/stability

Chest X-ray (CXR)

How it works: Low-dose radiography (PA/AP ± lateral).

Looks at: Pneumonia, effusion, pneumothorax, edema, cardiomegaly, lines/tubes.

CT Head (Non-contrast)

How it works: X-ray tomography without IV contrast.

Looks at: Intracranial hemorrhage, mass effect, hydrocephalus, skull fracture.

CT Pulmonary Angiography (CTPA)

How it works: Contrast-enhanced CT timed for pulmonary arteries.

Looks at: Pulmonary emboli, right-heart strain signs, alternative thoracic pathology.

CT Abdomen/Pelvis

How it works: Contrast or non-contrast per indication.

Looks at: Appendicitis, obstruction, diverticulitis, perforation, renal stones, AAA.

RUQ Ultrasound

How it works: Sonography of gallbladder/biliary tree.

Looks at: Cholelithiasis, cholecystitis, CBD dilation.

MRI Brain ± MRA

How it works: MR sequences (DWI/FLAIR); angiography if vascular.

Looks at: Acute ischemia, demyelination, posterior fossa lesions, aneurysms/stenosis.

Cardiovascular

structure • function • perfusion

Transthoracic Echocardiogram (TTE/POCUS)

How it works: Cardiac ultrasound.

Looks at: LV/RV function, wall motion, pericardial effusion/tamponade, valves.

BNP/NT-proBNP

How it works: Serum peptide with ventricular stretch.

Looks at: Supports heart failure diagnosis and severity.

Coronary CT Angiography (CCTA)

How it works: ECG-gated contrast CT of coronaries.

Looks at: Coronary stenosis/plaque for selected low–intermediate risk chest pain.

Neurologic

stroke • infection • pressure

Lumbar Puncture (CSF Studies)

How it works: Needle access to subarachnoid space.

Looks at: Cells, glucose, protein, opening pressure, culture/PCR for meningitis/SAH.

Electroencephalogram (EEG)

How it works: Scalp electrodes record cortical activity.

Looks at: Seizure activity, non-convulsive status, encephalopathy patterns.

GI / Hepatobiliary

pain • jaundice • infection

HIDA Scan

How it works: Nuclear tracer follows bile flow.

Looks at: Cystic duct obstruction/cholecystitis; GB ejection fraction.

Serum Lipase

How it works: Enzymatic assay.

Looks at: Pancreatitis (≥3× ULN with clinical/imaging correlation).

Liver Panel (AST/ALT/ALP/Bili)

How it works: Serum chemistries of hepatocellular/cholestatic markers.

Looks at: Hepatitis patterns, obstruction, biliary disease.

Vascular / Orthopedic

ischemia • fracture • infection

Arterial Doppler / CTA Limb

How it works: Duplex ultrasound or contrast CT of limb arteries.

Looks at: Acute limb ischemia level/severity; runoff; thrombus.

X-ray ± CT (Skeletal)

How it works: Planar radiographs; CT for complex/occult injury.

Looks at: Fracture, dislocation, alignment, hardware, osteomyelitis clues.

Compartment Pressure Measurement

How it works: Needle manometry in compartments.

Looks at: Compartment syndrome confirmation (delta-P with clinical exam).

Key Diagnostic Labs

specific • confirmatory

D-dimer (context-dependent)

How it works: Fibrin degradation product by immunoassay.

Looks at: If negative with low/intermediate pretest probability, helps exclude PE/DVT; positive → imaging.

ESR / CRP

How it works: Inflammation markers.

Looks at: Supports GCA, osteomyelitis, epidural abscess with imaging.

Serum Ammonia

How it works: Plasma assay; careful handling.

Looks at: Hepatic encephalopathy signal within clinical context.