Monitoring an individual’s health often requires assessment of serial laboratory results. Repeat results are seldom identical.
Changes in laboratory values may be due to biological variation, analytical imprecision, or a change in the individual’s condition. Sometimes it is difficult to decide if a change is medically significant. The magnitude of change that must occur in an analyte before the difference is considered medically significant can be calculated from the intra-individual variation and method precision.
The formula for calculating significant change between results is:
K(CVa2 + CVb2)1/2
K is a factor dependent on the probability level selected. A K value of 2.77 gives a probability level of less than 0.05. CVa is analytical imprecision and CVb is biological variation.
Using this formula, the magnitude of significant change was calculated for 31 of the most common laboratory tests.
|
Test |
Biological CV (%) |
Method CV (%) |
Significant Change % |
Test Result |
Equivalent Range |
|
ALT |
23.0 |
5.8 |
66 |
40 |
13-66 |
|
Albumin |
3.0 |
3.1 |
12 |
4 |
3.5-4.5 |
|
Amylase |
10.0 |
5.3 |
31 |
70 |
48-92 |
|
AST |
8.0 |
2.2 |
23 |
40 |
31-49 |
|
Bilirubin |
18.0 |
5.0 |
52 |
0.7 |
0.35-1.05 |
|
Calcium |
2.0 |
1.9 |
8 |
9.5 |
8.8-10.2 |
|
CO2 |
6.0 |
10.5 |
34 |
27 |
18-36 |
|
CEA |
10.0 |
4.2 |
30 |
3 |
2.1-3.9 |
|
Chloride |
1.0 |
1.0 |
4 |
102 |
98-106 |
|
Cholesterol |
6.0 |
2.0 |
18 |
200 |
165-235 |
|
HDL |
7.0 |
4.3 |
23 |
35 |
27-43 |
|
CK |
32.0 |
3.1 |
89 |
130 |
14-246 |
|
Creatinine |
5.0 |
0 |
14 |
1 |
0.86-1.14 |
|
Ferritin |
10.0 |
3.7 |
30 |
70 |
49-91 |
|
Glucose |
8.0 |
2.2 |
23 |
90 |
69-111 |
|
GGT |
14.0 |
2.0 |
39 |
50 |
30-70 |
|
Iron |
20.0 |
1.7 |
56 |
100 |
44-156 |
|
LDH |
13.0 |
1.8 |
36 |
450 |
286-614 |
|
Lipase |
14.0 |
1.5 |
39 |
170 |
104-236 |
|
Magnesium |
3.0 |
3.4 |
13 |
1.7 |
1.5-1.9 |
|
Phosphorus |
7.0 |
2.2 |
20 |
3.5 |
2.8-4.2 |
|
Potassium |
5.0 |
2.4 |
15 |
4.3 |
3.6-5.0 |
|
PSA |
18.0 |
3.1 |
51 |
4 |
2.0-6.0 |
|
Sodium |
0.80 |
0.6 |
3 |
141 |
137-145 |
|
TSH |
20.0 |
5.1 |
57 |
3.3 |
1.4-5.2 |
|
T4 |
6.0 |
3.2 |
19 |
1.2 |
1.0-1.4 |
|
Transferrin |
2.0 |
2.5 |
9 |
294 |
268-320 |
|
Triglyceride |
22.0 |
2.8 |
61 |
200 |
77-323 |
|
T3 |
8.5 |
4.2 |
26 |
145 |
107-183 |
|
Urea |
10.0 |
3.3 |
29 |
26 |
18.4-33.6 |
|
Uric acid |
9.0 |
1.4 |
25 |
5.1 |
3.8-6.4 |
|
Hematology |
|
|
|
|
|
|
Hemoglobin |
2.6 |
0.6 |
7.4 |
15 |
1.1 |
|
MCV |
1.1 |
0.7 |
3.6 |
90 |
3.3 |
|
WBC |
10.7 |
1.9 |
30.1 |
10,000 |
3,010 |
|
Platelet count |
9 |
1.9 |
25.5 |
300,000 |
76,440 |
The column labeled, “Significant Change” lists the percent that each test must change before different results are considered medically significant. These values were calculated at the 95% confidence level. The third column gives a hypothetical test result and the fourth column provides the range of equivalent results. Results falling within this range are not significantly different from the original result. For example, if a patient had an initial cholesterol level of 200 mg/dL and a subsequent level of 175 mg/dL this magnitude of change has a 95% likelihood of not being medically significant.
References:
Ross JW, Lawson NS. Analytic goals, concentration relationships and the state of the art for laboratory precision. Arch Pathol Lab Med 1995; 119:495-513
Fraser, CG. Biological variation in clinical chemistry. Arch Pathol Lab Med 1992;116:916-23
Fraser CG et al. Variation of common hematologic laboratory quantities in the elderly. Am J Clin Pathol 1989;92:465-70.
Fraser CG. The application of theoretical goals based on biological variation data in clinical chemistry. Arch Pathol Lab Med 1998;112:404-415.
Skendzel LP, Barnett RN, Platt R. Medically useful criteria for analytic performance of laboratory tests. Am J Clin Pathol 1985;83:200-205.
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