How Much Can A Laboratory Test Change And Still Be The Same?
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 in a laboratory result really reflects a change in health status. The magnitude of change that must occur before the difference is considered medically significant has been calculated for the most common chemistry, hematology and lipid tests.
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 for a healthy person and the fourth column illustrates how much a subsequent result would have to change to be considered medically significant. For example, if a patient’s initial hemoglobin was15 g/dL a subsequent level would have to change more than 1.1 g/dL to be considered medically significant.
|
Chemistry Test |
Significant Change % |
Example Test Result |
Significant Change |
|
ALT |
66 |
40 |
26 |
|
Albumin |
12 |
4 |
0.5 |
|
Amylase |
31 |
70 |
22 |
|
AST |
23 |
40 |
9 |
|
Bilirubin |
52 |
0.7 |
0.4 |
|
Calcium |
8 |
9.5 |
0.7 |
|
CO2 |
34 |
27 |
9.0 |
|
CEA |
30 |
3 |
0.9 |
|
Chloride |
4 |
102 |
4 |
|
CK |
89 |
130 |
35 |
|
Creatinine |
14 |
1 |
0.1 |
|
Ferritin |
30 |
70 |
21 |
|
Glucose |
23 |
90 |
21 |
|
GGT |
39 |
50 |
20 |
|
Iron |
56 |
100 |
56 |
|
LDH |
36 |
450 |
164 |
|
Lipase |
39 |
170 |
66 |
|
Magnesium |
13 |
1.7 |
0.2 |
|
Phosphorus |
20 |
3.5 |
0.7 |
|
Potassium |
15 |
4.3 |
0.7 |
|
PSA |
51 |
4 |
2 |
|
Sodium |
3 |
141 |
3.9 |
|
TSH |
57 |
3.3 |
1.9 |
|
T4 |
19 |
1.2 |
0.2 |
|
Transferrin |
9 |
294 |
26 |
|
T3 |
26 |
145 |
38 |
|
Urea |
29 |
26 |
7.6 |
|
Uric acid |
25 |
5.1 |
1.3 |
The degree of variability observed in hematology tests over time is inversely correlated with the lifespan of the three hematopoietic cell lines. Red blood cells circulate for 120 days and have the smallest variability, while white blood cells
survive only a few days and have the highest variability. Platelets have an intermediate lifespan of 7 days and have slightly lower variability than white blood cells.
|
Hematology Test |
Significant Change % |
Example Test Result |
Significant Change |
|
Hemoglobin |
7 |
15 |
1.1 |
|
Hematocrit |
9 |
45 |
4.1 |
|
RBC count |
6 |
5,000,000 |
311,000 |
|
MCV |
3 |
90 |
3 |
|
Total WBC |
30 |
10,000 |
3,000 |
|
Granulocyte % |
20 |
60 |
12 |
|
Lymphocyte % |
30 |
30 |
9 |
|
Monocytes % |
31 |
6 |
2 |
|
Platelet count |
26 |
300,000 |
78,000 |
Studies on the biological variability of hematology and chemistry tests are performed on healthy ambulatory patients. Hospitalized patients may experience even greater shifts in laboratory tests due to changes in posture, activity levels, diet, fluid balance, and medications.
Lipid and lipoprotein concentrations vary during the normal course of daily activity. Studies have demonstrated that within person variability is sufficient to make an individual move in and out of the predefined risk categories defined by the National Cholesterol Education Program (NCEP). As many as 11% of patients would be mistreated if risk assessment were made on the basis of a single lipid panel. NCEP guidelines acknowledge this variation by stressing that patient management decisions should be based on an average of at least two results.
|
Lipid |
Result (mg/dL) |
Significant Change |
Cholesterol |
180 |
32 |
|
|
200 |
35 |
|
|
220 |
39 |
|
|
240 |
43 |
|
|
260 |
46 |
|
|
280 |
50 |
|
|
300 |
53 |
HDL |
25 |
7 |
|
|
30 |
8 |
|
|
35 |
10 |
|
|
40 |
11 |
|
|
45 |
12 |
|
|
50 |
14 |
|
|
55 |
15 |
|
|
60 |
16 |
|
|
65 |
18 |
|
Triglycerides |
100 |
84 |
|
|
150 |
126 |
|
|
200 |
168 |
|
|
250 |
210 |
|
|
300 |
252 |
|
|
350 |
293 |
|
|
400 |
336 |
|
|
450 |
378 |
|
|
500 |
420 |
Cholesterol concentration must change at least 18%, HDL cholesterol 27% and triglycerides 84% before one can be assured that the difference is not simply due to intra-individual and analytical changes.