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Individual Biological Test Variation

One analytical goal that each laboratory should strive for is to make method imprecision for each analyte equal to or less than one half of the average intra-individual biological variation. Intra-individual variation is the amount that an analyte fluctuates within an individual over time. The degree of fluctuation is essentially the same irrespective of the time span of repeat measurements, instrumentation or methods used for measurement, geographic region, or patient’s age. Patients with stable chronic diseases (such as renal failure, hypertension, diabetes mellitus, and liver disease) may have different set points for an analyte, but the degreee of fluctuation about the set point is the same. Because of its constancy, intra-individual biological variation data can be used by any laboratory to evaluate method performance. The following table provides estimates of average intra-individual biological variation for the most common chemistry tests. 

Analyte

CVi (%)

Analyte

CVi (%)

ALT

23.0

FSH

2.9 m, 31 f

Albumin

3.0

GGT

14.5

Aldosterone

29.0

Glucose

8

ALP

8.0

Glucose, fasting

4.5

Amylase

10.5

17OHprogesteron

10 m, 20 f

Anion Gap

9.5

IgA, G, M

5

ACE

12.5

Insulin

15

Apolipo A1

6.5

Iron

20

Apolip B

7.5

LD

13

AST

8.2

Lipase

14

Bicarbonate

5.6

Magnesium

3

Bilirubin, T

18.0

Osmolality

1

Bilirubin, D

26

Phosphate

7

CA-125

25

Potassium

1.1

Calcium

3.0

Prolactin

7 m, 41 f

CEA

11.5

PSA

18

Chloride

1.2

Proteins, total

3

Cholesterol, T

6.0

Sodium

0.8

Cholesterol, HDL

7.0

Testosterone

8 m, 8 f

Cholinesterase

5.4

TSH

20

Cortisol

15

T4, total

7

CK, total

32

T4, free

6

Creatinine

5

Transferrin

2.3

DHEA-S

1.2 m, 5.6 f

Triglycerides

22

Estradiol

22 m, 61 f

T3, total

8.5

Ferritin

10

Uric acid

8.6

Fructosamine

4.6

Urea

10

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