CBC Beyond the Basics

 

The CBC report contains a lot of information.

Let us break it down and try to make some sense of it.

 

Most of the important information comes from only 4 of the tests:

  1. THE WBC COUNT
  2. THE HEMOGLOBIN LEVEL
  3. THE HEMATOCRIT LEVEL
  4. PLATELET COUNT
If these 4 tests are normal it is highly likely that any other abnormal CBC result is not of importance. 
 
The sections below provide more information about each CBC with differential test component.

THE WBC COUNT

 ELEVATED (HIGH) WBC COUNT

  • White cells are infection-fighting cells, part of your immune system.
  • A high WBC count suggests but does not diagnose, an infection.
  • An elevated WBC count is very non-specific and can be caused by many things which are not infections. 
  • Examples of things that can make the WBC count high include traumatic accidents, allergic reactions, inflammation unrelated to infections, and medications, particularly steroids. 
  • It is difficult to assign meaningful cutoff groups for mild, moderate, or severely elevated WBC counts. 
  • Roughly speaking, high WBC counts to about 13,000 are mildly elevated, up to about 20,000 moderate, and above this severe. 
  • But these have little meaning. Some common infections like strep throat can cause WBC counts above 20,000.  On the other hand, sometimes severe life-threatening infection occurs with low, normal, or mild-moderate elevations. 
  • Blood disorders, such as leukemia, become a concern with WBC counts over 40,000 that do not improve after other possible causes are treated.

LOW (OR NORMAL) WBC COUNT

  • While a normal WBC count may lessen the chances of infection, it definitely does not exclude the possibility. Many patients with severe infections have normal, or even low, WBC counts. A normal WBC count does not exclude appendicitis. 
  • Low WBC counts occur with some viral infections.  
  • Some normal, healthy, people always have a mildly low WBC count. 
  • Low and Critically low WBC counts can result from certain medications, particularly chemotherapy, or with bone marrow disorders.
  • Critically low levels can predispose to a severe infection.

 

 

HEMOGLOBIN AND HEMATOCRIT 

 
(Rough categories for adults)
 Normal RangemildmodSevere Critical

 Men:     HCT

                Hgb

37-51 

13-17.7

30-37

10-13

25-30

9-10

 18-24.9  

6.1-8.9

<18

<6.1

 Women: HCT

Hgb

34-47

 11-16

30-34

9-10.9

25-29

7-8.9

18-24.9

6.1-6.9

<18

<6.1

 

LOW HEMOGLOBIN (Hgb) AND HEMATOCRIT (Hct)

  • These two tests almost always move in the same direction, high or low, and we can consider them together. 
  • Red blood cells (RBC’s) contain a protein called hemoglobin (Hgb) which carries oxygen from your lungs to the rest of your body, where oxygen is used (to keep you alive!). 
  • Hgb is a measure of the amount of this protein in your blood. 
  • Hct measures the fraction of your blood which is RBC’s. 
  • If your Hgb is low you are anemic. Most of the time your Hct will also be low. 
  • Mild anemia can be normal for some people due to common
    genetic traits.
  • Not all anemia is from bleeding. If your bone marrow is not making enough hemoglobin and RBC’s, you will gradually become anemic. This can occur from deficiency of iron or other vitamin deficiencies or many other causes. 
  •  Besides acute or chronic bleeding, more common causes of anemia include chronic kidney disease, chronic infections or any type of chronic inflammation.  There are many others. 
  • Symptoms, if any, from anemia depend on more than just the Hgb or Hct level.
  • These factors include how fast the level fell from your normal level and your conditioning (heart, lung, muscles). 
  • Anemia may cause shortness of breath with exertion, general weakness, low blood pressure, and feeling lightheaded,  particularly when standing up suddenly. 

ELEVATED (HIGH) Hgb and/or Hct

  • High blood levels of hemoglobin or hematocrit usually cause no symptoms and require no treatment, unless extremely elevated.
  • Dehydration is a common cause of elevated Hgb or Hct.
  • Smokers, people with chronic lung diseases such as COPD, and people living at high altitudes (where the air has less oxygen) often have higher than normal levels. 
  • When levels are critically high people are at risk for abnormal blood clots which can cause strokes, heart attacks, deep vein thrombosis, and visual disturbances.
  • Critically high levels may indicate a serious bone marrow disorder, polycythemia vera. 
 
 

PLATLETS

 
Here are some things to keep in mind before interpreting a platelet count:
  • The categories of low and high platelets listed below are rough
    estimates.
  • There is a lot of overlap between specific causes of a high or low PLT count and the severity of the abnormality. Thus mild, moderate, or severe does not tell you much about the cause.
  • Laboratory issues may interfere with automated (machine) platelet measurements causing false low results. Usually, the first thing to do when an unexpected abnormal result occurs is to REPEAT THE CBC.
  • There are many causes of low platelets.
    • It may happen as a reaction to severe infections or other serious illnesses and thus not be primarily a platelet disorder.
    • as a side effect or reaction to many medications
    • alcohol abuse.
    • cirrhosis
    • immunologic disorders, bone marrow diseases.
That said:
Rough categories for platelet abnormalities.  (100,000= “100K)
 Normal RangeMildModerateSevereCritical
LOW

150k

to

450k

100k-150k50K-100k10k- 50k< 10k
HIGH450k-600k600k-800k800k-1 million>1000k(>1 million/ug)
 
 
 
 

MILDLY OR MODERATELY LOW PLATELETS

  • Mildly and Moderately low PLT counts are relatively common.
  • MIld or Moderately low platelets do not require specific treatment to correct the level to normal.
  • Although not dangerous, this should not be ignored as it may be a clue to some other problem. It should be monitored to ensure levels return to normal or don’t become lower.
  • Moderately low levels are more likely to be caused by a specific platelet disorder, but can still occur from any other cause listed above.

SEVERELY LOW PLATELET COUNTS

  • This does not cause bleeding.
  • However, bleeding from another cause, such as an injury, may happen easily, be hard to control, and require transfusion of platelets.
  • Treatment to increase the platelet level may be required before any planned surgery or procedures.
  • Testing to determine the cause of severely reduced platelets, if not already known,  is important.

CRITICALLY LOW PLATELETS

  • At these levels, bleeding can occur with minimal trauma or with no obvious injury.

  • Platelet transfusions are still not usually given in the absence of bleeding.

  • Critically low values are often seen after chemotherapy, with severe bone marrow disorders, and with some immune disorders.

HIGH PLATELET LEVELS

  • MILD, and  MODERATELY elevated platelet levels are common.
  • Iron deficiency anemia is a common cause.
  • Chronic Inflammation from infections and many other causes may elevate platelet levels. These are “reactive” causes of high platelets (reactive thrombocytosis).
  • People who have had their spleen removed or who have a spleen that is not functioning well, may have high platelets.
  • CRITICALLY ELEVATED platelet levels (over one million x 109/L) may cause blood clots. Paradoxically this can also predispose to bleeding as the platelets may not function properly.
  • CRITICALLY ELEVATED levels can occur as reactive thrombocytosis, but the possibility of a serious blood or bone marrow disorder must be considered.
  • In general, if there are no symptoms from high platelets, no specific treatment is needed to lower the PLT level.
  • If the levels don’t return to normal with the treatment of other conditions, like an infection,  then a blood specialist (hematologist) should be consulted to determine the cause of high platelets.

MPV- MEAN PLATELET VOLUME

  • This is just what its name implies, the average volume of your platelets.
  • Mild abnormalities in the absence of other abnormalities can
    probably be ignored. High MPV may indicate heredity platelet abnormalities or certain blood cancers.
  • In the latter case, there would likely be many other severe
    abnormalities on the CBC.
  • Few doctors, outside of blood specialists, pay much
    attention to this test.  

 MORE ABOUT PLATELETS

  • When your blood vessels are damaged by an injury or inflammation, platelets become “activated” and stick to the wall of the vein or artery.
  • Along with your clotting factors, they help build up the clot so you stop bleeding.
  • But activation of platelets from inflammation can be dangerous and causes clots where you don’t want them.
  • This can lead to clots in your heart or brain arteries and cause heart attacks and strokes.
  • For this reason, many people take “blood thinners” of the anti-platelet type.
  • They work by decreasing the activation of platelets, hopefully stopping the formation of clots in the wrong place.
  • If you are on anti-platelet medications the platelet count on your CBC should be normal. They don’t change the number of platelets, only their function.

RBC INDICES TESTS

General Considerations

  • These are tests that describe the size, shape and contents of the RBC’s.
  • When the hemoglobin or hematocrit are abnormal these results
    give hints as to the cause of the abnormality.
  • If the Hgb and HCT are normal abnormalities of the RBC indicies are of little or no importance.
  • If you are anemic (low Hgb and HCT), for whatever reason, it is likely that some of these will be abnormal. 
  • Abnormal RBC indices alone do not require any treatment.  
  • Common genetic variants can cause values to fall outside of the normal range. 
 

MCV- MEAN CORPUSCULAR VOLUME

  • This is the average volume of your red cells (corpuscles). 
  • A low MCV means the RBCs are, on average, smaller than normal.
  • The most common cause for this is iron deficiency but it also occurs with some common genetic variants such as
    Thalassemia.
  • A high MCV can occur with vitamin B12 or folate deficiencies.
  • There are many other causes of an abnormal MCV.

MCH- MEAN CORPUSCULAR HEMOGLOBIN

  • This is the average amount of oxygen-carrying hemoglobin in each cell.
  • In the absence of significant anemia, abnormalities generally
    require no further investigation or treatment.
  • Helps the clinician characterize the type of anemia.

MCHC- MEAN CORPUSCULAR HEMOGLOBIN CONCENTRATION

  • This is the average concentration of hemoglobin in the red cells:
  • It requires no specific treatment and an abnormal result poses no danger.
  • If your Hgb and HCT are normal this is essentially unimportant information.

RDW- RED CELL DISTRIBUTION WIDTH

  • In general, all your red cells are about the same size.
  • This test measures the variation in size of your red cells.
  • It is abnormally high in many types of anemia and sometimes helps us determine
    what is causing the anemia. 
  • Otherwise, an abnormal result is not unimportant. 

RETIC- RETICULOCYTES

  • Reticulocytes are immature RBC’s
  • They are released from the bone marrow into your blood when the body needs to rapidly replace lost blood. 
  • Elevated RETIC counts in the setting of acute blood loss are normal and mean you are replacing lost blood.
  • If your RETIC count does not go up when your Hgb is low, it could be due to a vitamin deficiency, iron deficiency or a bone marrow disorder. 
  • Retics are not part of the standard CBC or diff and must be specifically ordered as a separate test.

NRBC: NUCLEATED RED BLOOD CELLS 

  • These are immature blood cells that are not usually
    present. The normal value is 0/100WBC
  • There are many causes of elevated NRBC levels including bone marrow diseases, blood cancers, low blood oxygen levels, and severe anemia.
  • People who have had surgical removal of their spleen or a non-functioning spleen may show these cells.  
  • If NRBC’s are present the cause should be investigated if not already known. 

WBC DIFFERENTIAL TESTS (Diff)

General considerations:

  • The WBC count measures all of the different types of WBC’s, combined.
  • The CBC Differential (Diff) reports each sub-type of WBC separately.
  • Results are reported in 2 ways:
    • 1) The Percentage of each cell type (of the total WBC’s).
    • 2) The Absolute (ABS) count.  This is the concentration of each type.
  • The ABS count is equal to (WBC count) X (Percent of the type). 
  • THE BOTTOM LINE IS:
    • If your total WBC count is elevated, looking at percents informs on which cell types are mostly responsible. A higher than normal percentage of neutrophils and BANDS. is suggestive (not diagnostic) of a bacterial infection.
    • The ABS counts tell if you have critically low levels of any of the WBC cell types. This could indicate a bone marrow problem (blood cells are made in the bone marrow).
  • If your total WBC count is even mildly abnormal it is very likely that some of your Diff results will be abnormal. 
  • Likewise higher percents of LYMPHS or MONO’s are more suggestive of viral infections. 
  • Isolated non-critical abnormalities on the diff are very common on an otherwise normal or mildly abnormal CBC, and of no importance. 

WBC Diff: Understanding the distribution of cell types

  • Neutrophils and Lymphs are the predominant cell types on a typical CBC.
  • Low levels of Mono’s, Baso’s, Eos or an occasional Band form may be present. 
  • It is important to look at the relative percentage of each cell type and how they differ from normal. 
  • On a percentage basis, the typical range of their distribution is. 
NEUTROPHILS   BANDSLYMPHOCYTESMONOCYTES
50%-70%  0%18% to 42%2% to 11%
  • Bacterial infections tend to change this distribution, increasing the percentage (and absolute numbers) of Neutrophils and may cause the appearance of BANDS.
  • BANDS are an immature form of Neutrophil which shows up with more severe bacterial infections when extra help is needed to fight infection.
  • Viral infections tend to increase the relative percentages of LYMPHS or MONO’S.   
  • CRITICALLY LOW Neutrophil levels predispose to severe bacterial infections. 
  • CRITICALLY LOW Lymphocytes predispose to serious viral and parasitic infections. 

Here are a few examples:

 1 ) WBC count 12  (Diff: Neutrophil 90%, Band 5%, Lymph 5%,  Mono 1%)

This suggests a bacterial infection because the % of Neutrophils is higher than normal and Bands are elevated.

 

 2) WBC count 14 (Diff: Neutrophils 30%, Bands 0%,Lymph 69%,Mono 1%)

This looks more like a viral infection with a high percentage of Lymphocytes.

 

 

WBC CELL TYPES 

 

LYMPHOCYTES (LYMPH)

 

HIGH LYMPHS

  • Mild abnormalities of the lymphocyte counts are common and usually unimportant. 
  • Mild to moderate elevations suggest but do not diagnose, a viral infection.
  • But this can also be seen as part of a non-specific elevation of the WBC count from any type of physiologic stress and from some medications. 
  • CRITICALLY elevated levels or persistently high MILD-MODERATE levels should raise suspicion for chronic leukemias. 

LOW LYMPHS

  • Some perfectly healthy people have MILD-MODERATE  low lymphocyte counts all the time.
  • Low Lymphs are sometimes the result of medications given to try and suppress the immune system. These are given to organ transplant patients and those with many types of auto-immune disorders such as Lupus, MS, Psoriasis, Ulcerative Colitis, or Rheumatoid Arthritis. 
  • CRITICALLY low lymphocyte counts however can be an
    indicator of severe immune suppression.
  • CRITICALLY low lymph counts predispose to certain types of infections such as fungal infections, parasitic infections and viral infections. 

MONOCYTES (MONO)

  • This is another sub-type of the infection-fighting WBC population. 
  • Elevated levels are associated with certain viral infections, for example, mononucleosis. 

EOSINOPHILS (EOS)

  • Eosinophil levels are often elevated in response to an allergic reaction and
    may also be a response to a parasitic infection.
  • Absent (zero) or lower than normal levels is of no importance.

BASOPHILS (BASO)

  • Basophils levels may become elevated in response to an acute allergic reaction, acute and chronic infections as well as non-infection-related chronic inflammation.
  • The absence of BASO’s on your CBC is of no importance.

IMMATURE WBC FORMS: BANDS, METAMYELOCYTES, MYELOCYTES AND PROMYELOCYTES

  • Besides a low level of BANDS, these are usually absent (0%, ABS=none) on the CBC Diff.  They will usually not be listed on your report if none are detected.
  • During a severe infection, or with some bone marrow disorders, immature WBC’s make it out of the bone marrow (where they first develop) and may be detected in your blood.
  • An elevated BAND count raises concern for a serious bacterial infection. This is true even if the total WBC is normal and the ABS BANDS are elevated.
  • METAMYELOCYTES, MYELOCYTES, AND PROMYELOCYTES- These are even more immature WBC forms than band cells (as above).  If present they are very concerning for a severe bacterial infection or certain types of blood cancer. Their presence should not be ignored.