Lab Testing in Hematology Flashcards
16 year old homeless female presents to Hennepin County Whittier Clinic requesting birth control.
• She says that she feels well and has no significant past medical history
• Should you perform any laboratory tests?
Yes!
Order a full CBC w/ diff and maximize the value by knowing what all the numbers mean.
What are key points to remember regarding automated CBC?
- Even if you only order a hemoglobin, in most (but not all) labs the machine will perform the CBC and store the data
- If the lab uses a machine that performs everything (including automated differential, reticulocyte count, blood smear) you can request these tests later (within 24 hours!) and have them reported WITHOUT re-drawing the patient
- If the lab uses a machine that only performs what was ordered based on the bar code label, you can add tests on to the already drawn tube of blood if you do it within 24 hours of the blood draw. In the Epic system you request the full CBC with or without the automated differential as an “Add On”
What is the difference in cost between Hgb alone vs CBC vs CBC w/ diff? How does this compare to Hgb drawn with new tube for CBC w/ diff?
Hg $8
CBC $12
CBC w/diff $14
Hemoglobin with new tube drawn for CBC/ differential is $8 + $14= $22
What is the take home message regarding the cost of a CBC vs hgb?
- It costs the laboratory the same amount of money to perform and report a hemoglobin, or CBC or a CBC with automated differential count
- If you “add on” the additional tests within 24 hours the additional charge is negligible
- YOU add value by knowing what to look for and knowing what to do if any results are abnormal
What is RBC and how is it reported?
– Number of red cells in a measured volume of
blood
– Reported as number of cells per microliter or millions of cells per cubic milliliter
**
decreased RBC is called..
Anemia
Increased RBC means…
polycythemia/erythrocytosis
What is relative polycythemia?
typically 2/2 dehydration
What is the difference between primary and secondary absolute polycythemia?
Primary: Neoplastic proliferation of red cells
independent of erythropoietin = Polycythemia vera
Secondary: Anything that increases erythropoietin which means anything that decreases oxygen delivery to tissues (or an increase in erythropoietin not caused by hypoxia) 2/2 COPD, asthma, cigarette smoking
What is the pathophysiology of secondary polycythemia?
Increased EPO!
- Decreased O2 delivery to tissues
• Small red cells that don t deliver as much oxygen
• Normal sized or big red cells that aren t getting enough oxygen from the lungs
• Normal sized or big red cells that can’t get to the kidney because of constricted renal arteries - incresaed EPO from cytokine producing tumor or other source
What is a clue that small red cells that aren’t delivering as much oxygen may be causing secondary polycythemia?
low MCV
What causes artifactual polycythemia?
• Machine mistaking something else for red cells
• Something elses I have seen:
– Big platelets counted as red cells
– Parasites counted as red cells (usually plasmodium malariae)
What is an exogenous source that can cause in increase in RBC ?
exogenous EPO or doping!!
Increased RBC w/ high MCV think…
Neoplastic/external source of EPO
other drug effect?
inherited metabolic effect?
Increased RBC w/ normal MCV…
Hemoconcentrated?
O2 saturation?
EPO level?
19 yo who collapsed after a track meet:
hemoconcentrated from dehydration!
How is Hgb measured?
The machine lyses an aliquot of the specimen, sulfates the hemoglobin (to normalize the spectrum for all Hgbs - fetal, HbS, etc) and then measures the concentration using infrared absorbence.
How is Hgb reported?
gm/dl
Can a machine tell the difference between Hgb OUTside and hgb INSIDE RBCs?
NO
the machine itself hemolyzes RBCS to get the Hgb out
High Hgb think…
primary or secondary polycythemia
Low Hgb think…
ANEMIA
Hgbx3 should equal the…
Hematocrit! (ratio of total volume ofRBCs to total blood)
If (Hgb x3) - HCT is > +2 then…
there is either:
- free hemoglobin (lysed red cells) in the specimen,
- something is interfering with the hemoglobin measurement (bilirubin or very high TG) or
- red cells were sticking together in the machine
How is HCT calculated?
Total volume of RBCs/ total volume of plasma = HCT%
How is HCT calculated? Reported?
MCV (volume of RBC) x RBC/ sample volume….this is not as good as when they used to centrifuge it down
It is reported as a PERCENTAGE!
High Hct think…
usually polycythemia – lots of RBCs! (primary or secondary)
Low Hct think…
ANEMIA
What is the formula?
Hgb 14.9
Hct 38.5
14.9 x3 - 38.5 = 6.2
…if > 2 there is something WRONG
*HIGH MCV
MACROCYTIC RBC
*Normal MCV
normocytic RBC
*LOW MCV
microcytic RBC
What is MCV?
Mean cell volume measured for INDIVIDUAL red cells and reported in famptoliters
What is MCV useful?
can help guide evaluation of anemia…it SHOULD NOT be ignored if the patient is NOT anemic… you could miss B12 def
What should you know about MCV?
- It is AGE DEPENDENT and varies a lot in the first 10 years of lfie.
- In healthy adults the MCV will only change gradually unless there is massive hemolysis, massive pH or electrolyte shifts, transfusion or a SPECIMEN ERROR
- Since there is a wide range of normal for MCV a TREND can tell you as much as an abnormal MCV
What is a KEY POINT to know about MCV?
Normal ranges are age, gender and race dependent. If this exact same CBC is for a 2 year old Caucasian male, all of the red cell indices are NORMAL. EVEN IF THE MCV IS IN THE NORMAL RANGE, IF IT HAS CHANGED FROM THE PATIENT’S BASELINE IT SHOULD BE INTERPRETED AS ABNORMAL.
For this adult patient whose baseline MCV is 93.3 fl, an MCV of 83.8 fl is ______!! This trick is harder to use for infants and children at ages when the normal range for MCV changes.
Microcytic!!
a 2 mo F presents with FTT. What do you NEED TO KNOW before you continue her work up?
Whether or not she has been transfused!
The infant had been transfused with adult RBCs so the MCV is difficult to interpret. The child had been transfused, and the MCV shown is actually just the result of having adult red blood cells which are smaller than infant RBCs.
What is MCH?
mean corpuscular Hgb.. calcluated by the machine
not super useful
What is MCHC? When should you pay attention to it?
Mean corpuscular Hgb content
- Hgb gm/dl/Total red cell volume
- Reported in gm/dl
If it’s LOW
LOW MCHC
Hypochromic (decreased hemoglobin in the red cells)
NOrmal MCHC
Either normochromic or machine calculation lagging behind hypochromic morphology
High MCHC
hyperchromic or machine aftifact
31 yo M w/ h/o advanced cystic fibrosis:
RBC L
Hgb 8
MCV 75
MCHC L
This case showed that the low MCHC suggest the cause of the anemia was decreased production of hemoglobin rather than some other cause. This case will reappear and actually demonstrate a combination of iron deficiency anemia as well as anemia of chronic disease.
What is RDW?
red cell distribution width
- Calculated by the machine
- Coefficient of variation of red cell size
- Reported as %
- Used along with the MCV as a first step in evaluating microcytic anemia
37 yo M with lymphadenopahty
Hgb 17.3 H HCT 49.2 RBC 4.96 MCV 99.2 MCH 34.9 MCHC 35.2 RDW 12.4
2.7…hemoglobin/hematocrit discrepancy. It turned out they he had very elevated triglycerides and that was what was causing the elevation.
Key points for CBC summary slide
• The automated CBC is an inexpensive test that can provide lots of
useful information
• It is only useful if you think to order it
• It is only useful if you don t ignore the abnormal numbers
• It is only useful if you know how to interpret the abnormal numbers
• USE THE COMPUTER to look for baseline and evolution of changes in the CBC
• Even if you only order one count, some labs will have the entire CBC (including reticulocyte count) filed digitally and available for you (or the malpractice attorney) should it be requested
• If you “Add On” within 24 hours you avoid cost and discomfort of another blood draw
• If you don t understand the CBC results - Request a blood smear review by an experienced Pathologist
