About

Post-Void Residual (PVR) measurement is a standard non-invasive procedure used to assess urinary retention, bladder outlet obstruction (e.g., BPH), or neurogenic bladder. While dedicated automated bladder scanners exist, they are often unavailable in all wards. Manual calculation using a standard ultrasound machine is a reliable alternative when measuring the bladder's three dimensions: transverse, anterior-posterior, and superior-inferior.

This tool is optimized for point-of-care usage. It addresses the geometric variance of the bladder; while a full bladder approximates a sphere, a partially empty bladder typically collapses into a more triangular or cuboid shape. The "Advanced Mode" allows clinicians to toggle the correction coefficient (0.52 vs 0.7) based on institutional protocols or specific device calibration.

Formulas

The formula generalizes the bladder as a geometric shape:

Volume = L × W × H × C

Where:

C is the correction coefficient.
Standard Approximation (C = 0.52).
Alternative adjustment for instrument calibration (C = 0.7).

Reference Data

PVR Volume (mL)	Classification	Clinical Action (General Guide)
0 - 50	Normal	No action required. Healthy emptying.
50 - 100	Acceptable	Usually observing in elderly; generally normal.
100 - 200	Warning	Monitor. Repeat scan. Check for UTI or obstruction.
200 - 400	Retention	Intermittent catheterization may be indicated. Consult Urology.
> 400	Severe Retention	High risk of hydronephrosis/kidney damage. Immediate catheterization usually required.
Correction Factor	Shape Assumption	Usage Context
0.52	Ellipsoid	Standard US machines, full bladder.
0.72	Cuboid / Cylinder	Used by some bladder scan devices for non-spherical shapes.

Frequently Asked Questions

PVR should be measured immediately after the patient voids (preferably within 10-15 minutes). Delays allow renal output to refill the bladder, leading to falsely elevated residual readings.

The coefficient 0.52 (pi/6) assumes a perfect ellipsoid. However, historical data and certain portable scanner algorithms use a coefficient closer to 0.7 to account for the bladder flattening or becoming more cuboidal when it is not fully distended, ensuring retention is not underestimated.

The geometric principles apply, but the "Normal" reference ranges differ significantly for pediatric patients. For children, the formula (Age in Years × 30) + 30 is often used to estimate expected capacity, and PVR > 20ml is often considered abnormal. This tool's interpretation logic is calibrated for Adults.

This tool is designed for inputs in Centimeters (cm), which is the standard output for bladder calipers on ultrasound machines. If your machine measures in mm, divide by 10 before entering.