Blood Work Guide
BLOOD CELLS & COUNTS
The CBC checks three things: red blood cells, white blood cells, and platelets. Here is what each marker means, what shifts your results, and when a flagged value actually matters.
Medically reviewed by Missy Zammichieli, DNP, APRN, FNP-BC ยท Updated March 2, 2026
THE BOTTOM LINE
The CBC (complete blood count) checks three things: red blood cells (oxygen delivery), white blood cells (immune function), and platelets (clotting).
For most patients, the markers that actually matter are hemoglobin, hematocrit, WBC count, and platelet count. If you are on testosterone therapy, hematocrit is the safety marker your provider watches most closely.
Minor fluctuations are common and usually driven by hydration, recent exercise, or a mild illness — not something serious.
INTRODUCTION
About 55% of your blood is plasma. The other 45% is cells: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that stop bleeding. The CBC measures all three in a single draw.
It shows up on virtually every routine panel because it gives a broad snapshot of oxygen transport, immunity, and clotting from one tube of blood. For performance medicine patients, it also tracks how training, nutrition, hydration, and hormonal therapies affect blood cell production over time. Lab values require clinical interpretation — nothing here constitutes a diagnosis.
RED BLOOD CELLS
Red blood cells carry oxygen from your lungs to your tissues. They live about 120 days before being replaced. The red cell portion of the CBC tells your provider how many you have, how much oxygen-carrying protein they contain, and whether they are the right size.
Hemoglobin — The Headline Marker
Hemoglobin is the iron-containing protein inside red blood cells that actually binds oxygen. It is the primary measure of your blood’s oxygen-carrying capacity.
Reference range: ~13.0–17.5 g/dL (men); ~12.0–15.5 g/dL (women).
Low hemoglobin (anemia) means your tissues are not getting enough oxygen. Common symptoms: fatigue, exercise intolerance, feeling winded easily, pallor. The most common cause worldwide is iron deficiency.
High hemoglobin (polycythemia) thickens your blood and raises the risk of clots. It can be caused by chronic low oxygen levels, altitude, smoking, or testosterone therapy.
Anemia is not a diagnosis by itself — it is a finding that tells your provider to dig deeper into the cause.
Hematocrit — Percentage of Blood That Is Red Cells
Hematocrit measures what fraction of your total blood volume is occupied by red blood cells. It moves in lockstep with hemoglobin — roughly, hematocrit is about three times the hemoglobin value.
Reference range: ~38.3–48.6% (men); ~35.5–44.9% (women).
Because hematocrit is a percentage, it is sensitive to fluid levels. Dehydration concentrates red cells and raises hematocrit without any actual change in red cell production. Conversely, endurance-trained athletes often see lower hematocrit because their plasma volume expands — sometimes called “sports anemia,” which is actually a healthy adaptation, not true anemia.
If you are on TRT: Hematocrit is the primary safety marker your provider monitors. Testosterone stimulates red blood cell production, which can push hematocrit above 50% — a threshold where blood viscosity increases and clot risk rises. The Endocrine Society recommends checking hematocrit at baseline, at 3–6 months after starting therapy, and annually thereafter. If it exceeds 50%, your provider will typically discuss dose adjustment or therapeutic phlebotomy (blood donation).
Serial trends matter more than any single value. A hematocrit that climbs from 44% to 52% over six months on therapy is a clear signal, even if 52% is only slightly above the reference limit.
RBC Count, MCV, MCH, MCHC, and RDW — The Supporting Cast
These markers give your provider detail behind the headline numbers:
- • RBC Count is the raw number of red blood cells per microliter (men: ~4.5–5.5 million; women: ~4.0–5.0 million). On its own it tells you less than hemoglobin, but it helps complete the picture.
- • MCV (Mean Corpuscular Volume) is the average size of your red blood cells (reference: 80–100 fL). If your hemoglobin is low, MCV helps classify the type of anemia — small cells often point to iron deficiency, large cells to B12 or folate deficiency.
- • RDW (Red Cell Distribution Width) measures how much variation there is in red cell size (reference: 11.5–14.5%). An elevated RDW often shows up before MCV changes, making it an early warning sign for developing iron deficiency.
- • MCH and MCHC describe hemoglobin concentration per cell. Your provider uses them alongside MCV to pinpoint anemia type. They rarely drive decisions on their own.
Together, MCV and RDW help your provider classify anemia when hemoglobin is low. If your hemoglobin is normal, these supporting markers are unlikely to change your management.
PLATELETS
Platelets are small cell fragments that form the initial plug when you cut yourself. They circulate for about 8–10 days before being replaced.
Platelet Count
Reference range: 150,000–400,000 per microliter.
Low platelets (thrombocytopenia): Can result from decreased production (bone marrow issues, B12/folate deficiency, alcohol), increased destruction (immune conditions, medications, infections), or sequestration in an enlarged spleen. Bleeding risk generally goes up as counts drop below 50,000 and becomes significant below 10,000–20,000.
High platelets (thrombocytosis): Most commonly reactive — your body ramps up platelet production in response to infection, inflammation, iron deficiency, or surgery. This usually resolves on its own once the trigger clears.
Mild fluctuations are common and usually not significant. If a count comes back low and it does not match the clinical picture, your provider may recheck it — sometimes platelets clump in the collection tube and give a falsely low reading.
MPV (Mean Platelet Volume) is the average platelet size (reference: ~7.5–12.5 fL). Larger platelets are younger, freshly released from the bone marrow. Your provider uses MPV to help distinguish between production problems and destruction problems when the count is off.
WHITE BLOOD CELLS
White blood cells are your immune system’s workforce. The CBC reports a total count and then breaks it down into five subtypes.
WBC Count — Immune System Activity
Reference range: 4,000–11,000 per microliter.
High WBC (leukocytosis): Most commonly driven by infection, inflammation, stress, corticosteroid use, or recent exercise. A hard workout can temporarily double your WBC count for a few hours — this is normal and resolves on its own.
Low WBC (leukopenia): Can result from viral infections, bone marrow suppression, autoimmune conditions, medications, or nutritional deficiencies. Mild leukopenia is common and often benign. In people of African, Middle Eastern, and some Mediterranean descent, constitutionally lower WBC counts are a well-documented normal variant.
The total WBC count is a blunt instrument. A count of 12,000 could represent a bacterial infection or an allergic response. The differential is what tells the story.
The WBC Differential
Your WBC breaks down into five subtypes. Each responds to different threats. If any subtype is abnormal, your provider will flag it. The most common finding is a temporarily elevated count from a recent workout or mild illness.
| Cell Type | What It Fights | % of WBCs |
|---|---|---|
| Neutrophils | Bacteria, first responders | 40–70% |
| Lymphocytes | Viruses, long-term immunity | 20–40% |
| Monocytes | Cleanup crew | 2–8% |
| Eosinophils | Allergies, parasites | 1–4% |
| Basophils | Rare allergic responses | <1% |
In practice, the differential helps your provider distinguish infection types. Bacterial infections typically drive up neutrophils. Viral infections tend to push up lymphocytes. Elevated eosinophils in the context of respiratory symptoms suggest allergies rather than infection. These are patterns, not rules — exceptions are common, and your provider interprets them alongside your symptoms and history.
WHAT AFFECTS THESE RESULTS
Exercise timing. A hard training session can temporarily spike your WBC count by 50–100% for several hours. For the most accurate baseline, draw labs in a rested state — ideally 12–24 hours after intense training.
Hydration. Dehydration concentrates all blood cells, raising your RBC count, hemoglobin, and hematocrit without any real change in red cell production. Overhydration has the opposite effect. Standardize your hydration before a draw — well-hydrated but not water-loaded.
Iron status. Iron is required for hemoglobin production. When stores deplete, the progression is: ferritin drops first, then serum iron declines, then RDW rises, then MCV falls, and finally hemoglobin drops. Pairing the CBC with an iron panel (ferritin, serum iron, TIBC) gives a more complete picture.
Altitude. Living or training at altitude naturally raises hemoglobin and hematocrit through increased red blood cell production. Sea-level reference ranges may not apply above ~4,000 feet.
Medications. Testosterone therapy increases red blood cell production — this is an expected effect, not a surprise finding. Corticosteroids cause higher neutrophils and lower lymphocytes by redistributing cells. Metformin can lower B12 absorption over time, leading to larger red cells.
Inflammation and illness. Acute infection raises WBC and can suppress hemoglobin. Chronic inflammatory conditions can produce persistent mild anemia even when iron stores are adequate. Recent vaccination can transiently elevate lymphocytes.
Menstruation. Regular menstrual blood loss is the most common cause of iron deficiency in premenopausal women, which affects hemoglobin, hematocrit, and the red cell indices.
COMMON QUESTIONS
“My WBC is slightly low — should I be worried?”
In most cases, a mildly low WBC (e.g., 3,500–4,000) is clinically insignificant. Constitutional low WBC counts are common in certain populations, and viral illnesses can suppress the count for days to weeks. It becomes more concerning when it is a new finding, trending downward on serial labs, associated with low hemoglobin or platelets, or accompanied by recurrent infections. Trend matters more than a single number.
“Does exercise affect my CBC?”
Yes. High-intensity exercise can temporarily raise WBC by 50–100% for a few hours. Chronic endurance training can expand plasma volume, which dilutes hemoglobin and hematocrit — this looks like anemia on paper but actually reflects a healthy adaptation where total red cell mass is normal or increased. For the most representative results, draw labs at least 12–24 hours after intense training.
“Why is my hematocrit monitored on testosterone therapy?”
Testosterone stimulates red blood cell production through multiple mechanisms. This is a predictable, dose-dependent effect. The concern: when hematocrit rises above roughly 50–54%, blood viscosity increases, raising the risk of blood clots, stroke, and other cardiovascular events. The Endocrine Society recommends checking hematocrit at baseline, at 3–6 months, and annually on therapy, with dose reduction or phlebotomy considered when hematocrit exceeds 50%.
“What is anemia?”
Anemia means your hemoglobin is below the level needed for adequate oxygen delivery — less than 13.0 g/dL for men and less than 12.0 g/dL for non-pregnant women by WHO criteria. Iron deficiency is the most common cause worldwide. Symptoms include fatigue, weakness, shortness of breath on exertion, and reduced exercise capacity. Anemia is a finding, not a final diagnosis — the CBC indices (MCV, RDW) help narrow down the cause so your provider can treat it.
References
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This article is for informational purposes only and does not constitute medical advice. Blood work results should be interpreted by a qualified healthcare provider in the context of your complete medical history, medications, and clinical presentation.
UNDERSTAND YOUR NUMBERS
Your CBC is only as useful as the interpretation behind it. Get your blood work reviewed by a provider who looks at the full picture — hydration, training, medications, and trends over time.
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