Shear valve sampling technique for hematology analyzers

Accurate results start with precise blood sampling

The sampling technique is a critical part of laboratory measurements and is reflected in the accuracy of every result you report. An exact volume of blood needs to be precisely diluted in a highly repeatable manner.

Commonly used sampling techniques

Two different sampling techniques are commonly used in today’s hematology analyzers: (i) a micro-pipette connected to a vacuum-generating, step motor-controlled syringe pump or (ii) a rotating shear valve. The suction process of the syringe pump is often time-based, and the technology is considered less complex and therefore a more cost-efficient solution. However, a syringe doser is more sensitive to variable pressure conditions and requires a mechanism for moving the sample probe in horizontal and vertical directions.

In contrary, the rotating shear valve allows cutting an absolute volume for analysis and is not sensitive to altitude and other factors that might affect pressor conditions. On the other hand, shear valve technology is considered more complex and therefore a more expensive solution. Many shear valves also have a design that makes them sensitive to environmental impurities that might cause leakage and thus will require regular cleaning.

Boule shear valve technology

Every Boule Swelab Alfa Plus, Medonic M32 and Exigo H400 hematology analyzer comes equipped with a high-precision shear valve. Not only is the shear valve extremely precise to deliver accurate results, it is also maintenance-free for you to enjoy significantly reduced maintenance costs:

  • Closed design to prevent entry of environmental impurities that might cause contamination and leakage.
  • Flushed with Diluent to prevent build-up of salt deposits that might cause leakage.
  • Soaked in Diluent to avoid wear and tear of the shearing discs, ultimately mitigating the need for replacement.

Atmospheric pressure variations will not affect the blood cell count. A blood sensor prevents inaccurate results caused by air in the sample. When enabled, aspiration stops when blood is detected by the blood detector sensor. This functionality can be disabled by the operator to instead employ a fixed aspiration type.


A)      The blood sample is aspirated to the shear valve

B) The process is terminated by the blood detector.

C)      The shear valve cuts off an exact blood volume for the first dilution.

D) The second dilution for the WBC count and HGB measurement in the WBC measuring chamber

E)      The second dilution for the RBC ad PLT counts in the RBC measuring chamber

F)      Total dilution for  WBC and HGB is 1:400 and for RBC and PLT 1:40 000

Learn more

Read more about how Boule’s system design helps optimize clinical performance:

Swelab Alfa Plus analyzer design and maintenance procedures for optimized system performance

Medonic M32 analyzer design and maintenance procedures for optimized system performance

Exigo H400 analyzer design for optimized system performance