Z-Score Calculator for Laboratory Proficiency Testing

What is a Z-Score?

A Z-score is a standardized measure used in proficiency testing to evaluate how far your laboratory's result deviates from the expected value. It tells you:

  • How many standard deviations your result is from the consensus value
  • Whether your laboratory's performance is satisfactory
  • If corrective actions are needed
Z-Score Calculator
Enter your measured value
The assigned value from your PT provider
The target standard deviation from your PT provider
Z-Score Formula

Z = (x - μ) / σ

Where:
x = Your laboratory result
μ = Consensus value (robust mean)
σ = Standard deviation for proficiency assessment
Your Result Visualization
Understanding Your Z-Score
Performance Categories:
|Z| ≤ 2
Satisfactory
2 < |Z| < 3
Questionable
|Z| ≥ 3
Unsatisfactory
Practical Example:

A laboratory measures glucose in a PT sample:

  • Laboratory result: 5.8 mmol/L
  • Consensus value: 6.0 mmol/L
  • Standard deviation: 0.2 mmol/L

Z-Score = (5.8 - 6.0) / 0.2 = -1.0

This result is satisfactory (|Z| < 2)