**ABC/2**, also confusingly written as **1/2ABC** in some literature, is a fast and simple method for estimating the volume of intracerebral hemorrhage (or any other ellipsoid lesion for that matter) which does not require volumetric 3D analysis or software.

Intracerebral hemorrhage volume is an important predictor of morbidity and mortality (and thus trial eligibility) which is often under-reported ^{1}. ABC/2 has been well-validated and correlates highly with volumes calculated by planimetric techniques, although does tend to overestimate ^{2,3,7}.

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#### Formula

First described by Kwak et al. ^{4 }and popularized by Kothari et al. ^{2}:

- A x B x C / 2
- A = greatest hemorrhage diameter in the axial plane
- B = hemorrhage diameter at 90º to A in the axial plane
- C = originally described as the number of CT slices with hemorrhage multiplied by the slice thickness, but can simply be substituted with the craniocaudal diameter of the hemorrhage where there is access to multiplanar reformats
^{1}- this dimension is known as height (H) in other formulas

If the measurements are made in centimeters (cm), then the volume will be in cubic centimeters (cm^{3}) or milliliters (mL) (numerically equivalent). The volume does tend to overestimate the true volume, with errors increasing in more irregularly-shaped and larger hemorrhages ^{7}.

##### Mathematical basis

The above formula is a simplified version of the formula for the volume of an ellipsoid, which is:

- 4/3 π x (A/2) x (B/2) x (C/2)
- where A, B and C are the three diameters of the ellipsoid as defined above

If π (Greek letter pi) is approximated to 3, then the formula can be simplified to ABC/2 (in truth, π is an irrational number, starting 3.141...).

#### Interpretation

A baseline intracerebral hemorrhage volume of >50-60 mL is a poor prognostic marker ^{1,5}.

#### Alternatives

In addition to the ABC/2 formula, there are other formulas which have been described and studied to estimate the volume of intracerebral hemorrhage ^{7}:

- formulas using A, B, and C (as defined above):
- ABCπ/6 (or π/6ABC): the precursor formula (Tada formula) to ABC/2 where π is estimated as 3 instead
^{7} - ABC/3 (or 1/3ABC): was created to balance the over-estimation of ABC/2, but tends to under-estimate instead
^{7} - 2.5ABC/6 (or 2.5/6ABC): created to balance the over-estimation of ABC/2 and the under-estimation of ABC/3, has a greater accuracy than other formulas that use A, B, and C in one study
^{7}

- ABCπ/6 (or π/6ABC): the precursor formula (Tada formula) to ABC/2 where π is estimated as 3 instead
- formulas using the hematoma area in the maximally involved slice (S) and height (H or C):
- SH/2 (or 1/2SH): derived from ABC/2 with the background theory that S will be slightly less than A x B, found to be the most accurate formula in one study (in comparison to other formulas including ABC/2)
^{7} - SHπ/6 (or π/6SH): derived from ABCπ/6, and found to be the second-most accurate formula in one study after SH/2
^{7}

- SH/2 (or 1/2SH): derived from ABC/2 with the background theory that S will be slightly less than A x B, found to be the most accurate formula in one study (in comparison to other formulas including ABC/2)

#### Practical points

There are some pitfalls with the ABC/2 method:

- assumes an ellipsoid lesion (and thus the more the lesion deviates from this morphology the more inaccurate the calculated volume will be)
- overestimates oral anticoagulant-related intracerebral hemorrhage volumes (because they are often irregular in shape)
^{3}

To overcome these pitfalls, there are alternative formulas available which may have greater accuracy, such as SH/2 (or 1/2SH) and 2.5ABC/6 (or 2.5/6ABC) ^{7}.