How do you determine a positive shifting dullness and what if dullness on the right side becomes tympanic on the left?

Shifting Dullness Technique and Interpretation

Shifting dullness is positive when you percuss dullness in the flanks with the patient supine, mark the dullness-tympany interface, roll the patient to the opposite side, and observe the dullness shifting to the new dependent (lower) side—if dullness starts on the right flank when supine and becomes tympanic when the patient rolls left (with new dullness appearing on the left dependent flank), this confirms positive shifting dullness indicating at least 1,500 mL of ascitic fluid. 1

Proper Technique

The correct method requires a systematic approach:

• Start with the patient supine and percuss from the midline laterally toward both flanks 1
• Mark the point where percussion changes from tympany (resonant) to dullness on each side—this represents the fluid-air interface 1
• Roll the patient to one lateral decubitus position (e.g., left side down) and wait 30-60 seconds for fluid to redistribute by gravity 1
• Re-percuss the previously marked areas—positive shifting dullness means the area that was dull when supine becomes tympanic on the upper (non-dependent) side, while new dullness appears on the lower (dependent) side 1

Interpreting Your Specific Scenario

Your description of "dullness from right then tympanic going to the left" is exactly what positive shifting dullness should demonstrate: 1

• When supine, you percuss dullness in the right flank (fluid settling by gravity) 1
• When the patient rolls to the left side, that same right-sided area becomes tympanic (air-filled bowel floats up) 1
• Simultaneously, new dullness appears in the now-dependent left flank (fluid shifts down by gravity) 1

This represents classic positive shifting dullness and indicates at least 1,500 mL of free intraperitoneal fluid. 1, 2

Diagnostic Performance

Shifting dullness has 83% sensitivity and 56% specificity for detecting ascites, making it the most sensitive bedside physical examination technique available. 1, 2

Key clinical thresholds:

• Approximately 1,500 mL of fluid must be present before shifting dullness becomes detectable 1, 2
• If no flank dullness is present in the supine position, the patient has less than 10% probability of having ascites, effectively ruling out the diagnosis 1, 2
• Smaller volumes (<1,500 mL) require ultrasound, which can detect as little as 100 mL 2

Critical Next Steps After Positive Finding

When shifting dullness is positive (Grade 2 or 3 ascites), diagnostic paracentesis is mandatory to determine the underlying cause and exclude spontaneous bacterial peritonitis (SBP). 2

The urgency is real:

• Each hour of delay in diagnostic paracentesis increases in-hospital mortality by 3.3% in patients with suspected SBP 2
• SBP occurs in 10% of hospitalized cirrhotic patients and carries 20% in-hospital mortality despite treatment 2
• One-year survival after SBP is only 34% 2

Common Pitfalls to Avoid

Do not rely on shifting dullness in obese patients—physical examination is unreliable in obesity, and you should proceed directly to ultrasound rather than waste time with potentially false-positive findings. 1, 2 Thick layers of adipose tissue can mimic shifting dullness (pseudoascites), leading to futile paracentesis attempts. 3

Do not assume the absence of shifting dullness rules out all ascites—while negative findings make ascites unlikely (<10% probability), small-volume ascites (Grade 1) is only detectable by ultrasound. 1, 2

Do not delay paracentesis for coagulopathy correction—bleeding complications occur in only 0.2-2.2% of procedures with a death rate of 0.02%, so proceed with diagnostic paracentesis even with abnormal coagulation studies. 2