Bladder Outlet Obstruction Index Calculator
Bladder Outlet Obstruction Index Calculator
The Abrams–Griffiths number from a pressure-flow study — detrusor pressure at maximum flow minus twice the maximum flow rate — with the bladder contractility index from the same two numbers. These are the only two figures in this category that measure obstruction rather than infer it.
Bladder Outlet Obstruction and Contractility Indices
Pressure-flow study → BOOI and BCIA pressure-flow study with a detrusor pressure at maximum flow of 55 cmH2O and a Qmax of 9 mL/s
Two indices from the same two numbers
under 20 unobstructed · 20–40 equivocal · over 40 obstructed
BCI = PdetQmax + 5 × Qmax
under 100 weak · 100–150 normal · over 150 strong
In women: BOOI = PdetQmax − 2.2 × Qmax, with cut-offs 20 points lower
- the sign on the flow term is the whole difference
- the obstruction index SUBTRACTS twice the flow and the contractility index ADDS five times it, so at a fixed detrusor pressure they move in opposite directions as flow rises, and the gap between them is always exactly seven times Qmax. Both equal PdetQmax when the flow is zero. A transposed coefficient would be invisible at zero flow and nowhere else
- the two inputs are NOT interchangeable
- a detrusor pressure of 60 with a Qmax of 8 gives 44; entered the other way round it gives −112, which is at least obviously wrong. The subtler error is using the maximum detrusor pressure of the void instead of the pressure AT maximum flow, which inflates the index without looking wrong at all
- PdetQmax
- detrusor pressure is vesical pressure minus abdominal pressure, measured simultaneously, at the instant of maximum flow. Everything on this page depends on that subtraction having been done properly
- derived in men
- the original work enrolled mainly men over 50 with benign prostatic enlargement. Solomon’s 2018 optimisation for women uses 2.2 rather than 2 and cut-offs 20 points lower — obstruction above 20, no obstruction below 0 — and that figure is printed above so the men’s bands cannot be read onto a woman’s study. The men’s contractility index appears to overestimate contractility in women, and the IUGA review read for this page calls for research into the definition rather than offering a replacement
- what this page does not do
- it reports where a number falls among published boundaries. A score, an index, a measured volume or an attenuation value is not a diagnosis, and a proportion measured in a cohort is not a probability for one patient.
Worked example
A pressure-flow study with a detrusor pressure at maximum flow of 55 cmH2O and a Qmax of 9 mL/s
BOOI = 55 − (2 × 9) = 55 − 18 = 37.0
37 is inside the 20–40 equivocal range — the study has not separated obstruction from underactivity in this man
BCI = 55 + (5 × 9) = 55 + 45 = 100.0, which sits exactly on the boundary between the weak and the normal contractility categories
With Solomon's women's coefficient the index would be 55 − (2.2 × 9) = 35.2, and against the women's cut-off of 20 rather than 40 that is above it rather than equivocal — the same study read as two different findings depending on which published set applies
Note how little separates the readings: a Qmax of 7 instead of 9 gives a BOOI of 41 and moves the index out of the equivocal range altogether. Two millilitres per second is well inside the within-subject variation the EAU describes for Qmax
The published categories for both indices
| Index | Formula | Published boundaries |
|---|---|---|
| Bladder outlet obstruction index (BOOI), men | PdetQmax − 2 × Qmax | Under 20 unobstructed · 20–40 equivocal · over 40 obstructed |
| Bladder outlet obstruction index, women (Solomon 2018) | PdetQmax − 2.2 × Qmax | Over 20 obstruction · below 0 no obstruction — 20 points lower than the men’s set |
| Bladder contractility index (BCI) | PdetQmax + 5 × Qmax | Under 100 weak · 100–150 normal · over 150 strong |
| Bladder voiding efficiency, the third of the three indices | Voided volume ÷ (voided volume + residual) × 100 | No published category. See the voiding efficiency page |
What the non-invasive numbers can and cannot substitute for
| Non-invasive measure | Performance for obstruction | Source |
|---|---|---|
| Qmax of 10 mL/s | Specificity 70 per cent, positive predictive value 70 per cent, sensitivity 47 per cent | EAU male LUTS guideline, 2026 |
| Qmax of 15 mL/s | Specificity 38 per cent, positive predictive value 67 per cent, sensitivity 82 per cent. Above 15 mL/s the EAU states obstruction cannot be excluded | EAU male LUTS guideline, 2026 |
| Post-void residual of 50 mL | Positive predictive value 63 per cent, negative predictive value 52 per cent | EAU male LUTS guideline, 2026 |
| The IPSS, or any symptom questionnaire | The EAU cites a systematic review in which individual symptoms and questionnaires were not significantly associated with one another for diagnosing obstruction | the IPSS page |
| Voiding efficiency | No significant relationship with the obstruction index in 143 men | ICS 2025, abstract 565 |
The only number here that measures obstruction
Everything else in this category infers. A symptom score records bother, a prostate volume records size, a residual records what the bladder kept, and the published data say none of them identifies bladder outlet obstruction. The obstruction index measures it, because it is computed from a pressure-flow study: the detrusor pressure at the instant of maximum flow, minus twice that flow. Under 20 the published category is unobstructed, 20 to 40 inclusive is equivocal, and above 40 is obstructed.
The companion index uses the same two numbers with the sign and the weight of the flow term changed: the contractility index is the detrusor pressure plus five times the flow, with under 100 weak, 100 to 150 normal and above 150 strong. The pair exists because a low flow has two causes that look identical from the outside — a blocked outlet and a feeble detrusor — and only a simultaneous pressure measurement separates them. Note the arithmetic relationship between them: at zero flow both equal the detrusor pressure, they move in opposite directions as flow rises, and the gap between them is always exactly seven times Qmax. A coefficient transposed between the two would be invisible at zero flow and obvious nowhere else, which is why both are printed together here.
Three limits are worth carrying. The first is the equivocal range itself: a fifth of the index’s useful span is a zone in which an invasive test has not answered the question, and in the urogynaecological series read for this page the clinicians’ own obstruction diagnosis agreed with an index of 40 or more in 79.4 per cent of cases — one disagreement in five. The second is sensitivity to the flow measurement: a Qmax of 7 rather than 9 moves an index of 37 to 41, out of the equivocal range, and the EAU describes Qmax as prone to within-subject variation. The third is population. The boundaries were derived almost entirely in men over 50 with benign prostatic enlargement. Solomon’s 2018 work optimised the obstruction index for women as a 2.2 coefficient with cut-offs 20 points lower, and that figure is printed above so the men’s bands cannot be read onto a woman’s study; the men’s contractility index appears to overestimate contractility in women, and no replacement has been established.
The third index from the same 1999 editorial, bladder voiding efficiency, has its own page, and the symptom score the whole investigation usually starts from has another. The one thing that cannot be done here is to place a Qmax on a published reference nomogram: the Liverpool and Siroky centiles are curves read off a chart, this engine has no table interpolation, and a fitted stand-in for them would misclassify at exactly the extremes where a decision sits.
Frequently asked questions
What is the formula for the bladder outlet obstruction index?
BOOI = PdetQmax − 2 × Qmax, where PdetQmax is the detrusor pressure at the moment of maximum flow and Qmax is that flow in mL/s. It is also called the Abrams–Griffiths number. Under 20 is unobstructed, 20 to 40 equivocal and above 40 obstructed.
What is the difference between the obstruction index and the contractility index?
Only the flow term. The obstruction index subtracts twice the flow; the contractility index adds five times it. Both equal the detrusor pressure when the flow is zero, and the two always differ by exactly seven times Qmax. They answer different questions — whether the outlet is blocked, and whether the detrusor is working — and a low flow needs both.
Can these indices be used in women?
Not with the men’s boundaries. The original work enrolled mainly men over 50 with benign prostatic enlargement. Solomon and colleagues optimised the obstruction index for women in 2018 as PdetQmax − 2.2 × Qmax with cut-offs 20 points lower — obstruction above 20, no obstruction below 0 — and that value is shown under the headline. The men’s contractility index appears to overestimate contractility in women and no validated replacement was found.
Why is an index of exactly 40 equivocal rather than obstructed?
Because the published range is written “20–40 equivocal” and “over 40 obstructed”, so both boundaries of the equivocal band are inclusive. This page implements that exactly: 20.0 and 40.0 both read as equivocal, and 40.1 does not.
Do I need a pressure-flow study, or will a flow rate do?
The published numbers are in the second table. A Qmax of 10 mL/s has a sensitivity of 47 per cent for obstruction and at 15 mL/s a specificity of 38 per cent, and the EAU states that above 15 mL/s obstruction cannot be excluded. The guideline also notes that below 10 mL/s obstruction is likely and urodynamics is not necessarily needed, with no consensus above that.
What this page will not tell me
Every threshold, range and performance figure on this page is the published figure from the source named beside it, and each one depends on the population, the method and the equipment it was derived in. Where your own report, laboratory or local guideline gives a different figure, that figure governs. A score, an index, a measured volume or an attenuation value is not a diagnosis, and a proportion measured in a cohort is not a probability for one patient. This page reports a figure and what the published sources attach to it. It does not make a clinical decision and cannot.
Related calculators
References
- Abrams P. Bladder outlet obstruction index, bladder contractility index and bladder voiding efficiency: three simple indices to define bladder voiding function. BJU Int. 1999;84(1):14–15 — cited as the source of all three definitions in the International Continence Society abstracts read for this page; the editorial itself was not fetched.
- Nitti VW. Rev Urol. 2005;7(Suppl 6):14–21 — cited for the BOOI boundaries (under 20 unobstructed, 20–40 equivocal, over 40 obstructed) and the BCI boundaries (under 100 weak, 100–150 normal, over 150 strong) in the Bangladesh Journal of Urology article read for this page; the supplement itself was not fetched.
- International Urogynecological Association. Is there any role of bladder outlet obstruction index and bladder contractility index in women? IUGA Spotlight v17-5 — second independent statement of both formulas, the men’s cut-offs, and Solomon et al.’s 2018 women-specific coefficient.
- Could voiding efficiency predict bladder outlet obstruction or bladder contractility? International Continence Society 2025, abstract 565 — 143 men; voiding efficiency as VV / (VV + PVR) × 100, with the ICS-BOOI and ICS-BCI formulas and the ROC figures quoted on this page.
- European Association of Urology. Guidelines on the Management of Non-neurogenic Male Lower Urinary Tract Symptoms, 2026 edition — Diagnostic Evaluation. uroweb.org.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/