The International Continence Society (ICS) defines detrusor underactivity (DU) as “a contraction of reduced strength and/or duration, resulting in prolonged bladder emptying and/or a failure to achieve complete bladder emptying within a normal time span”. This definition is quantifiable by the non-invasive urodynamic parameters - voiding time (Vt), post-void residual (PVR) and voiding efficiency (void%). A well-established invasive method to grade detrusor voiding contraction is by the Schäfer pressure/flow nomogram (linearized passive urethral resistance relation), LinPURR. The goal of this study was to compare non-invasive parameters of DU with LinPURR contraction grading.

Pressure-flow studies (PFS) and uroflowrates from men with lower urinary tract symptoms were included. Patients with volume <100mL, abnormal urinalysis, neurological or congenital disorders, pelvic or radical prostate surgery, or with urethral stricture were excluded. We determined Vt, PVR and void% of uroflowrate and related these parameters to dichotomised LinPURR contraction grading. 
Thresholds of Vt <80 seconds, PVR <150 ml, and void% >80% were chosen.

327 PFS and uroflowrate studies were analysed. Void% had a sensitivity and specificity of 15-30% to predict urodynamic DU. Combining void% and Vt demonstrated a sensitivity of 6.7% and specificity of 59.5%; combining PVR and Vt a sensitivity of 6.9% and specificity of 64.5%.

When comparing noninvasive parameters void% was the most accurate single parameter to diagnose DU, performing better than a high PVR or a long Vt. Although 66.9% of patients with DU will be missed. 
Adjusting the thresholds did result in higher sensitivity values, but lower specificity values. 
The AUC values of void% as well as PVR and Vt were <0.62, suggesting that none of the parameters distinguished between normal contraction and DU well.
Uroflowrate is inevitably the result of contraction in combination with outflow resistance. This implies that for an accurate grading of contraction and a reliable diagnosis of DU, a PFS is mandatory. Nevertheless, void% and other uroflowrate parameters may be, or become, sensitive (and acceptably specific) parameters to evaluate the effect of therapy for an underactive bladder. 
As there is no curative therapy for DU yet, the clinical implication for grading contraction is merely to differentiate between DU and BOO in patients with LUTS. When a therapy for DU will be developed, grading may be used to select proper candidates and to do proper proof of principle studies.

The noninvasive parameters, based on the ICS definition for DU, are not predictive for DU as single parameter or in combination. Void% is the most predictive parameter.

Continence 2S2 (2022) 100249
DOI: 10.1016/j.cont.2022.100249