L3 Reference intervals

Question 1

For determining RI, outliers are excluded in the post-analytical phase.
For example, unhealthy individuals that are discovered during the test are excluded.
In addition, statistical modality can be done to establish a central 95%.
Briefly explain the Gaussian distribution and non-Gaussian distribution, as well as examples of applying them.

Answer 1

1. Gaussian distribution (normal distribution)
   - parametric method (+/- 2SD)
   - e.g. Na+, K+, Ca2+, Albumin
2. Non-Gaussian distribution
   - Non-parametric method (2.5-97.5 percentile)
   - e.g. Bilirubin, CK
   - i.e. 5% of normal patients may be outside RI

Question 2

What are clinical decision levels?

Give an example.

Answer 2

Range/threshold passing which a different medical decision is recommended.
e.g. TDM/ Cardiac troponin level

Question 3

Which type of cardiac troponins are used in PWH?

What is the cut-off?

Answer 3

cTnT rather than cTnI

Higher cutoff, one-tailed: 99th percentile with <10% coefficienct of variation (CV = SD/mean x 100%)

> lower sensitivity but high specificity

Question 4

What form of Vitamin D do we measure?

Answer 4

storage form - Total 25-OH vitamin D

bone-related disease is the only confirmed associated factor

Question 5

What are the 2 limitations of RI?

Answer 5

1. Analytical variation (SD1) - repeated measurements of a single specimen 20 times
2. Biological variation (SD2) - repeated measurements at regular intervals in a healthy individual

Question 6

What is the definition of a significant change from the previous result in repeated measurement of a specimen/ individual?

Answer 6

(Value2-value1) >2.77 SD

where SD = square root (SD1^2+SD2^2)

Question 7

A hypertensive male with plasma K+ of 5.5 mmol/L on admission. The next day, plasma K+ is 6.2 mmol/L. Is there a significant change?
For K+, the biological and analytical variations are 0.10 and 0.19 mmol/L, respectively.

(3 marks)

Answer 7

1. SD = sqaureroot (SD1^2+ SD2^2)
   = squareroot (0.1^2 + 0.19^2)
   = 0.22
2. 2.77 x 0.22 = 0.61 mmol/L
3. 6.2-5.5 = 0.7 mmol/L
   where 0.7 > 0.61, thus there has been a significant change in plasma potassium