Percussion provides a gentle means of localizing abdominal tenderness, and of differentiating between solid and gas filled structures, thus of defining the borders of solid organ.

Commence with percussion of the four quadrants (figure 24a–d), locate the lower border of the liver (figure 25a) and spleen (figure 25b) the bladder and uterus (figure 25c), and any dullness in the flanks. If the latter is detected it is further assessed for shifting dullness (see below).

To look for tenderness, percuss all four quadrants leaving any known tender area till last. The vibration of gentle percussion is sufficient to produce pain from a sensitive peritoneum (percussion rebound). This form of localization is much less painful for the patient than defining tenderness by superficial or deep palpation or by rebound tenderness (page 310). The technique is particularly useful in children. Always watch the patient’s face while undertaking percussion or any of the palpation techniques described below.

When defining borders of organs and masses, percuss from resonant (gas filled) to dull (solid organ). The liver edge is located by percussing sequentially from the right iliac fossa to the costal margin in the right midclavicular line. The dullness is usually located at the costal margin but a very large liver may extend down to the right iliac fossa. Enlargement is expressed in the number of fingers or hand-breadths below the costal margin. The upper edge of the liver usually reaches the fourth right intercostal space, percuss downwards in the midclavicular line from the second or third interspace. Hyper-resonant lung, such as in emphysema and pneumothorax, and a pneumoperitoneum, can make it difficult to locate the dullness of the upper margin.

The normal spleen is sited posterior to the left midaxillary line and is not easily detected by percussion. It enlarges across the abdomen towards the right iliac fossa. Percussion for splenic dullness therefore starts in the right iliac fossa, passes across the umbilicus to the left costal margin at the anterior axillary line, and then along the tenth rib posteriorly. Percussion from the umbilicus down to the symphysis is usually resonant but dullness may be encountered from the upper border of an enlarged bladder, uterus or ovary, coming out of the pelvis.

Abdominal masses are generally dull to percussion and alter normal patterns of resonance.

Ascites produces dullness to percussion in the flanks when lying in the supine position (figure 26a). Percuss from resonant to dull, moving the left hand so that the fingers remain parallel to the dull edge being sought. Note the fluid level and then rotate the patient 45 degrees to each side in turn (figure 26b). The abdominal wall fluid level changes since the fluid surface remains horizontal, a phenomenon known as shifting dullness. Fluid may be drawn off (paracentesis), the needle is inserted away from the epigastric vessels; subumbilical or left iliac are common sites (figure 27a,b).