April 2005, Page 38

Interpretation of Blood Spatter for Defense Attorneys - Part 1
By Louis L. Akin

áA basic understanding of the principles and procedures of blood spatter analysis will allow the defense attorney to intelligently converse with the defense blood spatter expert or cross-examine the prosecution’s expert regarding the blood evidence and the expert’s conclusions. Fortunately, these principles and procedures are not complicated or complex and can be learned from a source as brief as this article. This understanding is important, because the interpretation of blood spatter patterns and other evidence at crime scenes may reveal critically important information to the defense team such as:

1. The positions of the victim, assailant, and objects at the scene.

2. The type of weapon that was used to cause the spatter.

3. The number of blows, shots, stabs, etc. that occurred.

4. The movement and direction of victim and assailant, after bloodshed began.

5. It may support or contradict statements given by witnesses.1

The crime scene reconstructionist may use the blood spatter interpretation to determine:

1. What events occurred

2. When and in what sequence they occurred

3. Who was, or was not, there

4. What did notáoccur

The lists of precisely what information can be learned by the interpretation of blood stain patterns are similar for Bevel and Gardner
2, James and Eckert3, Hueske4, Slemko5, and Sutton6.

Questions of Interpretation: How Exact is Exact?

There are a few things to bear in mind when preparing a defense that includes blood spatter interpretation or when preparing to cross-examine the prosecution’s expert.

Blood spatter analysis, much like profiling, fingerprinting, or wilderness tracking, is an applied science or art rather than a hard science.

• Different textures of surfaces upon which the blood impacts may cause vastly different stain patterns resulting in spines or satellites or irregularly shaped patterns. The patterns may also be affected by the distance the drop fell, by repeated drops in one place, and by the direction and velocity of the drop as it contacted the surface.

• The classifications of the velocities of blood spatter are arbitrary and grew out of convenience based on experience at crime scenes.

• The judgment as to the velocity of the spatter is based on the majority of the stains present in an area.

• The crime scene and all photographs should be carefully scrutinized to see if the interpretations are plausible, or if other interpretations may be equally plausible.

• When interpreting cast-off blood, the accepted practice is to add one blow to the total of cast-off patterns, the reasoning being that with the first blow the instrument would have been dry.

• Adult males have about 5 to 6 liters of blood volume.

• Adult females have about 4 to 5 liters of blood volume.

• Loss of greater than 1.5 – 2 liters of blood volume may result in shock and immobilization.

Blood is slightly more viscous than water and this viscosity (about 4.5 as compared to water which is 1.0) helps form the blood into a spherical shape as it travels freely through the air.7

The Theory: The Teardrop vs. The Blood Drop

Figure 1: Fountain in Denver Airport

Experiments with blood have shown that blood tends to form into a sphere rather than a teardrop shape when free falling or projected in drop size volumes (approximately 0.05ml or 20 drops per milliliter; though some are larger and some are smaller.)8 The formation of the sphere is a result of surface tension.9 Fresh blood is slightly more viscous than water because of its components: corpuscles, leukocytes, platelets, salt, and collagens, and so it tends to hold the sphere shape in flight. The sphere shape of blood in flight is critical to the calculation of the angle of impact of blood spatter which will be used to determine the point or area from which the blood originated or point of origin (PO)10, 11

Using Pythagorean Trigonometry To Determine The Angle Of Impact, Point Of Convergence, And Point Of Origin

Hypotenuse2 = Opposite2 + Adjacent2 (H2 = O2 + A2)

Generally, a single spatter of blood is not enough to determine the point of origin at a crime scene. The determination of the angle of impact and placement of the PO should be based on the consideration of a number of spatters. The process for determining the angle of impact is not complicated. When a sphere of blood strikes a flat surface the diameter of the sphere in flight will equal the width of the stain on the surface (which is equal to the opposite side of a right triangle) as seen in Figure 1. The length of the spatter will be equal to the hypotenuse of an inverted triangle. This is portrayed graphically in Figures 2, 3, and 4.

Figure 2: Side view of blood drop in air, and then striking a flat surface

Figure 3: Top View of drop in air and the stain left after it has hit a flat surface

Figure 4: Resulting stain left by drop of blood that impacted at about 30 degrees of a flat surface

To find the point of origin (PO), one must first determine the two dimensional point of convergence (POC). The POC is the intersection where lines drawn through the center of the individual stains meet (at the X-Y axis intersection) and is determined by drawing lines or strings through the long axis of individual spatters as seen in Figure 5 below.

Figure 5: Lines through the central axes of the spatter cross at the point of convergence

The next step in the process is to determine the angle of impact (AOI) for representative bloodstains. A stated above opposite and hypotenuse are the same as the length and width. The AOI will necessarily be the same as angle a inside the triangle (as in Fig 2a). The angle of impact is the inverse arc sin of the W/L ratio, so first calculate ratio (W/L) then SIN -1 (2nd function) to get the degrees of aáand AOI.

To Determine the Angle Of Impact (AOI).

i. Where Opposite = Width and Hypotenuse = Length

ii. Angle of Impact = Inverse of arc sin of W/L ratio

a. First calculate ratio (W/L)

b. Then SIN -1 (2ndákey) to get degree of AOI.

For purposes of visual illustration to the jury, strings can be used to demonstrate the angles of impact and point of convergence as shown below.

Figure 6: Lines drawn through the central axes of the blood drops can be laid flat to show where they converge as in Figure 5, and then raised as in this picture to show points of origin. In this photograph the POC would be at the XYZ axis on the floor and the PO near the upper arrow

The Perpendicular Or Z Axis

Once the point of convergence and the angles of impact have been established, the next step is to locate the point of convergence three dimensionally, which will require a perpendicular axis that is ordinarily dubbed Z. The Z axis is the same as the long axis of a standing person. Where the strings or imaginary lines of the trajectories of the blood stains meet on the Z axis will approximate the point or area from which the blood was disgorged, dubbed the point of origin. See Figure 7 above.

Figure 7: The perpendicular or Z Axis

While the point of convergence is the two-dimensional intersection of the X and Y axes the point of origin (PO) is located above the POC on the perpendicular axis 90 degrees perpendicular to the floor. It is the point from where the blood origin or was disgorged from the body. In the diagram below, the PO is determined by calculating the angle that the blood struck the wall and drawing a line from each blood spot back to the Z axis.

Figure 8: Lines converge in the third dimension at the PO on the Z axis

The formula to determine the point of origin on the perpendicular (Z) axis is similar to the one used to establish the angle of impact except that the TAN function of the calculator is used. First, measure the distance from each blood stain along its central (Y) axis to the POC. Second use the calculator to obtain the TAN of the degrees AOI. Third, multiply the TAN of the AOI by the distance measured along the Y axis.

Determine Point Of Origin On The Perpendicular (Z) Axis

For each representative bloodstain:

Where distance from bloodstain to AOC = Y axis
Tan of 0 (AOI) x Y = PO

AOI Where O = 1.5mm and H = 3mm, then ratio = .05
Take -SIN 0.5 and the 0 = 30░
Tan of 30░ = 0.57735
And where length of Y axis (distance to POC) is 90cm then
PO = Tan of 0 (30░) x Y (90cm)
Calculate as Tan (30░) = 0.57735 x 90cm = 51.961

Answer: PO = 52cm
Then 52cm / 2.54 = 20.4724409 inches
Measure 52cm (or 20.5 inches) from the floor (X-Y axis) up the Z axis and you will arrive at the approximate point of origin or point where the blood left the body.


1. James, Stuart H, Eckert, William G. Interpretation of Bloodstain Evidence at Crime Scenes, 2nd Edition, CRC Press 1999 p10-11.

2. Bevel, Tom; Gardner, Ross M. Bloodstain Pattern Analysis, 2nd Ed. CRC Press 2002.

3. James, Stuart H, Eckert, William G. Interpretation of Bloodstain Evidence at Crime Scenes, 2nd Edition, CRC Press 1999.

4. Hueske, Edward E., Shooting Incident Investigation/Reconstruction Training Manual, 2002.

5. Slemko, J. Bloodstain Pattern Analysis Tutorial Forensic Consulting.

6. Sutton, Paulette T., Bloodstain Pattern Interpretation, Short Course Manual, University of Tennessee, Memphis TN 1998.

7. Bevel, Tom; Gardner, Ross M. Bloodstain Pattern Analysis, 2nd Ed. CRC Press 2002.

8. James, Stuart H. Eckert, William G., Interpretation of Bloodstain Evidence at Crime Scenes, 2nd Edition, CRC Press 1999 p20.

9. Sutton, Paulette T., Bloodstain Pattern Interpretation, Short Course Manual, University of Tennessee, Memphis TN 1998 Principle 2áp 4.

10. This author prefers to use the term point of hemorrhage, but point of origin is the accepted term.

11. Although most experts use the word point, the word area would be a more prudent and conservative one to use. n

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