Kruger's Strict Criteria

Definition - What does Kruger's Strict Criteria mean?

Kruger's strict criteria are a standard for measuring sperm morphology, or its shape. The Kruger criteria are considered more in-depth and critical than the standard “crude morphology” assessment completed during a basic sperm analysis. The process is typically completed in men who have sperm that appears to be visually competent, but, along with a partner, are struggling with unexplained infertility.

FertilitySmarts explains Kruger's Strict Criteria

When undergoing Kruger's strict criteria assessment, sperm is examined under a microscope that magnifies the sample to 1000x. Any minor deformity in the sperm's shape or structure is enough to classify the sperm as abnormal according to the Kruger's strict criteria.

A standard sperm contains four distinct components:

  • An oblong head, where the DNA is stored
  • A thick middle body called the midpiece, which houses the mitochondria. The mitochondria acts like the sperm's engine, giving it the power to move
  • A tail
  • An acrosomal cap, which sits at the tip of the head, and aids in fertilization

Any abnormalities of these features are enough to classify the sperm as damaged. Examples of abnormalities include:

  • Abnormally sized heads
  • Double heads
  • Missing or disfigured midpiece
  • Double tails
  • Short tails
  • Kinked tails
  • Missing acrosomal cap
  • Disfigured acrosomal cap

The final percentage of entirely undamaged sperm determines the resulting Kruger's strict criteria score. High scoring sperm is more likely to contribute to pregnancy, whether introduced to an egg during sexual intercourse or through assisted reproductive treatments (ART), including in vitro fertilization (IVF).

Strict morphology scores and their indications:

  • Over 14% normal – high fertility potential
  • 4-14% normal – somewhat impacted fertility potential
  • 0-3% normal – severe impairment, possibly unable to fertilize through sexual intercourse, and may require fertility treatment

In addition to Kruger's strict criteria, the other widely used sperm analysis criteria are produced by the World Health Organization (WHO). The WHO follows a similar scale, and as of 2010 also considers 4% and higher of normal sperm as the advantageous composition for fertility.

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