Hematology Tests

Important hematology tests:

Osmotic fragility test:
This is an important test to understand and can be a bit counterintuitive. The basis of the test is heparinized blood is put into a spectrum of salt solutions and tested for hemolysis. For normal RBCs, initial hemolysis is at 0.45% NaCl, and complete hemolysis at 0.30%. What does this mean? When RBCs are added to high salt concentrations, they lose water and become wrinkly (crenated). When RBCs are added to low salt solutions, they take in water until they equilibrate to the outside salt concentration or until they burst. So for normal RBCs to all burst, the salt concentration needs to be pretty low (0.30%). RBCs that have weaker cell walls or are more “fragile” will burst in higher concentrations of salt. For example, spherocytes will show initial hemolysis at 0.65% NaCl (meaning some will burst) and complete hemolysis at 0.45% NaCl (meaning all will burst).

The osmotic fragility test will be increased in hereditary spherocytosis, hemolytic disease of the newborn, and acquired hemolytic anemia.

The osmotic fragility test will be decreased in thalassemia major, sickle cell disease, and when target cells are present.

Erythrocyte Sedimentation Rate (ESR):
The ESR is a common hematology test that is used because it is quick, inexpensive, and can be of clinical utility. The ESR basically measures how quickly RBCs settle. Things that will cause an elevated ESR are rouleaux (clumped cells fall faster), increased amounts of fibrinogen, macrocytosis, and increased immunoglobulin. A tilted tube and/or increased room temperature can also cause an increased ESR.

Things that can cause a decreased ESR are: hemolysis, excess anticoagulant, decreased room temperature, bubbles in the column, microcytosis, and polycythemia.

Cold agglutinin test:
A decreased RBC count with an increased MCV, MCH, and MCHC could be due to autoagglutination. When a patients RBCs stick together in vitro, the automated counter may count them as one large cell. This will also lower the RBC count since multiple RBCs are being counted as one. Another thing to note is hematocrit on automated cell counters is calculated in part by multiplying the RBC count by the MCV. (This makes sense if you look back to our MCV calculation above). So since the hematocrit is calculated using the two parameters just outlined as being incorrect, the hematocrit will be incorrect and low in this case. A decreased hematocrit will give you an increased MCHC.