Graded compression. Find the appendix, prove it is blind-ending, then judge diameter, compressibility and the fat around it.

The appendix arises from the caecum at the convergence of the three taenia coli, and it is a BLIND-ENDING tube. Blind-ending is what separates it from small bowel — you must see the tip end and not continue, in real time. Anything you cannot follow to a blind end is not proof of the appendix.
It has no peristalsis and no valvulae conniventes. A loop that peristalses is bowel. Terminal ileum is the commonest impostor and it moves.
Position is highly variable: retrocaecal, pelvic, subhepatic, or left-sided in situs inversus and malrotation. "Not seen in the right iliac fossa" is not "not inflamed" — it usually means it has not been looked for everywhere.

High-frequency LINEAR probe (7–12 MHz); switch to curvilinear only for depth in a large patient.
GRADED COMPRESSION is the technique: apply slow, steady pressure over several seconds to displace bowel gas. Pressing suddenly makes the patient guard and defeats the whole examination.
Find the psoas muscle and iliac vessels as your posterior landmark, then walk medially from the caecum along the taenia.
If nothing is found: LEFT lateral decubitus brings a retrocaecal appendix forward; scan the pelvis with a full bladder; check subhepatically.
In a female, examine the ovaries and adnexa in the same sitting — the main alternative diagnosis.

Identify the caecal pole, follow the taenia to the appendiceal origin, and track the tube to its BLIND END. A loop you cannot follow to a blind end is not proof of the appendix.
Confirm it does not peristalse — terminal ileum is the commonest impostor and it moves.
Measure the OUTER-TO-OUTER transverse diameter on true cross-section (normal up to 6 mm).
Test COMPRESSIBILITY with the probe: a normal appendix flattens completely, an inflamed one does not.
Assess the periappendiceal FAT — inflamed fat is echogenic and non-compressible. This is what turns a borderline diameter into a diagnosis.
Look for an appendicolith: echogenic focus with clean posterior shadowing.
Elicit maximal tenderness under the probe over the structure you have identified, and say so explicitly.
Look for perforation: loss of wall continuity, a periappendiceal collection, free fluid.

A blind-ending, non-compressible, aperistaltic tube over 6 mm outer diameter WITH inflamed surrounding fat and probe tenderness at that exact point.
The conjunction is the diagnosis. Diameter alone over-calls: a 6–8 mm compressible appendix with clean fat is commonly normal.

⚠️ A NORMAL-LOOKING ULTRASOUND DOES NOT EXCLUDE APPENDICITIS. Non-visualisation is a non-diagnostic study, not a negative one, and must be reported as such with a recommendation — never as "no evidence of appendicitis".
To call it excluded on ultrasound you must have SEEN the whole appendix to its blind end, fully compressible, under 6 mm, with clean surrounding fat.
On CT, a normally-filled appendix with clean fat is a genuine exclusion.

Reporting "appendix not visualised" without saying that this is a NON-DIAGNOSTIC study rather than a negative one.
Calling terminal ileum the appendix — it peristalses and is not blind-ending.
Measuring inner-to-inner, or on an oblique section, both of which under-read.
Judging compressibility without enough patient cooperation to press properly.

Acute appendicitis

Reference values