What Causes Low Red And White Blood Cells? | Cause List

Seeing two “low” flags on a lab report can make your stomach drop. Still, a low red blood cell measure and a low white blood cell measure don’t point to one single disease. The same pattern can come from a short-lived issue, a medicine reaction, a nutrient gap, or a bone marrow disorder that needs fast attention.

This guide lays out the most common causes, the clues that help sort them, and the next steps clinicians often take. It’s meant to help you show up prepared, not to replace medical care.

Low Red And White Blood Cells Causes Doctors Check First

Most people learn about low counts after a complete blood count, often called a CBC. A CBC reports red cells, hemoglobin, white cells, platelets, and extra details like red cell size.

When both red and white cells run low, clinicians often start with three broad ideas: the bone marrow is making fewer cells, the body is losing or breaking down cells faster than it can replace them, or cells are being held in an enlarged spleen.

Cause Group	How Counts Drop	Clues That Often Show Up
Drug side effects	Marrow slows production	Drop starts after a new drug or dose shift
Cancer therapy	Chemo or radiation injures marrow cells	Counts dip after treatment cycles, then rebound
Marrow failure disorders	Marrow can’t produce enough cells	Low platelets may join in; bruising, fatigue
B12 or folate deficiency	Cells don’t mature normally	Large red cells (high MCV), mouth soreness
Serious infection	White cells get used up; marrow may pause	Fever, chills, rapid decline in how you feel
Immune-driven cell loss	Immune system targets blood cells	Rashes, joint pain, autoimmune history
Enlarged spleen	Spleen traps cells	Fullness under left ribs, liver disease history
Kidney disease	Low erythropoietin lowers red cell output	Low hemoglobin plus kidney lab changes
Marrow crowding from blood cancers	Abnormal cells take up marrow space	Abnormal smear, swollen nodes, night sweats

What Low Counts Can Feel Like

Symptoms depend on which cell line is low and how fast it changed. A slow decline can be quiet; a fast decline tends to hit hard.

Signs linked to low red blood cells

• Low energy that doesn’t match your sleep
• Shortness of breath with stairs or a brisk walk
• Fast heartbeat or a pounding pulse
• Lightheadedness when standing
• Pale skin or pale inner eyelids

Signs linked to low white blood cells

• Frequent infections or infections that linger
• Fever without a clear reason
• Mouth sores, gum irritation, or sore throat
• Skin infections that start from small cuts

When Low Counts Need Same-Day Care

If your white cell count is low, infection can turn serious fast. Seek same-day care if you have any of the following.

• Fever at or above 38°C (100.4°F), or shaking chills
• Shortness of breath at rest, chest pain, or fainting
• New confusion, severe weakness, or trouble staying awake
• Bleeding that won’t stop, black stools, or vomiting blood
• Rapidly spreading skin redness, or painful urination with fever

If you’re receiving chemotherapy or taking immune-suppressing drugs, don’t wait on a fever. Your care team may have a specific plan for urgent testing and treatment. The MedlinePlus Complete Blood Count (CBC) test page explains what’s measured and why it’s ordered.

What Causes Low Red And White Blood Cells? Main Mechanisms

People often ask, what causes low red and white blood cells? Two counts tend to drop together when something affects cell production or survival in a broad way. These are the main mechanisms clinicians use when they build a differential.

Lower production in the bone marrow

Bone marrow makes red cells, white cells, and platelets from stem cells. When the marrow slows down, more than one count can fall.

Medication-related marrow suppression

Many medicines can lower white cells, red cells, or both. Some antibiotics, anti-thyroid drugs, seizure medicines, and immune-suppressing therapies can do this in a predictable, dose-related way. Other drugs cause a rare reaction that appears without warning. Timing matters, so bring a dated list of all prescriptions, supplements, and injections.

Don’t stop a prescription on your own. A clinician weighs risks, benefits, and safer alternatives while keeping an eye on the trend.

Cancer treatment effects

Chemotherapy targets fast-growing cells, so marrow gets hit along with cancer cells. Radiation can also reduce counts, especially when treatment fields include marrow-rich bones like the pelvis. Dips often line up with treatment cycles, then counts rise as the marrow rebounds.

Marrow failure and marrow disorders

Aplastic anemia is a marrow failure condition where the marrow can’t make enough new blood cells. It can be linked to immune injury, certain infections, and drug reactions. The NHLBI page on aplastic anemia describes how marrow failure can lower multiple blood cell types.

Other disorders include myelodysplastic syndromes, myelofibrosis, and blood cancers that begin in marrow. A blood smear can show abnormal cell shapes or immature cells, which may lead to bone marrow testing.

B12, folate, copper, and mixed deficiencies

Vitamin B12 and folate are needed for normal cell division. When they’re low, the marrow may release fewer cells, and red cells may be larger than usual. Copper deficiency is less common, yet it can lower white cells and cause anemia.

Iron deficiency usually causes anemia without low white cells. If both are low, clinicians often search for a second driver, like a drug effect or marrow disorder.

Faster loss or breakdown of cells

Sometimes the marrow is working, yet cells are being destroyed or lost quicker than they can be replaced.

Immune conditions

In some autoimmune diseases, the immune system attacks blood cells or their precursors. Clues can include rashes, joint pain, mouth ulcers, and past immune diagnoses. Lab work may show markers that point toward immune activity.

Infections

Many infections raise white cells, yet some viruses can lower them, and severe infections can use white cells faster than the marrow can replace them. If fever, chills, and a sudden decline appear together with low counts, urgent evaluation is wise.

Bleeding and hemolysis

Bleeding lowers red cells. White cells don’t usually fall from bleeding alone, so a low white count alongside anemia pushes the workup wider. Hemolysis, where red cells break down early, can cause anemia with jaundice or dark urine.

Cells held in an enlarged spleen

The spleen filters blood and clears old cells. When it enlarges, it can hold onto red cells and white cells, leaving fewer in circulation. This is often tied to liver disease, some infections, and certain blood cancers. On exam, a clinician may feel the spleen below the left rib cage.

How Clinicians Narrow The Cause

A CBC is a snapshot. Most workups rely on trends plus a small set of add-on tests that point toward a cause group.

Repeat CBC with a white cell differential

A repeat test checks if the result was a one-off glitch and shows whether counts are stable, falling, or trending up. The differential breaks white cells into types. A low absolute neutrophil count is the biggest driver of infection risk.

Red cell indices and reticulocytes

MCV shows red cell size. Reticulocytes are young red cells. Low reticulocytes suggest low production; high reticulocytes lean toward blood loss or red cell breakdown.

Peripheral smear

A smear lets the lab report cell shapes and immature forms. That can point toward marrow disorders, vitamin deficiency patterns, or hemolysis clues.

Test	What It Checks	Why It’s Often Ordered
Repeat CBC with differential	Trend plus white cell types	Confirms pattern and flags neutropenia
Reticulocyte count	Marrow output of red cells	Separates low production from loss or breakdown
Peripheral smear	Cell appearance	Hints at marrow disorders or vitamin patterns
Vitamin B12 and folate	Nutrients for cell division	Finds macrocytic anemia causes
Iron studies	Iron stores and transport	Helps sort iron deficiency vs chronic illness
Kidney and liver tests	Organ function	Links anemia to low EPO or spleen trapping
Bone marrow testing	Marrow cells and structure	Confirms marrow failure, dysplasia, or cancer

Questions That Make Your Appointment Easier

Bring your lab printout or a screenshot.

• Which number is low: red cell count, hemoglobin, or both?
• Which white cell type is low, and what is my absolute neutrophil count?
• Are platelets low too?
• Do my indices point toward small, normal, or large red cells?
• When should I repeat the CBC, and what change would trigger urgent action?
• Which medicines on my list can lower counts, and what are the alternatives?
• Which tests are next, and what will each one rule in or rule out?

Checklist For The Next Two Weeks

While you wait for repeat labs or follow-up, these steps can reduce risk and help your clinician connect symptoms to the CBC trend.

1. Write down fevers, sore throats, mouth sores, or new cough, with dates and temperatures.
2. Track bruising, nosebleeds, black stools, or heavy menstrual bleeding.
3. Keep a dated list of all pills, supplements, and injections you take.
4. Wash hands often and avoid sharing drinks, especially if neutrophils are low.
5. Skip raw seafood and undercooked meats if your clinician said your infection risk is high.
6. Seek care the same day if fever starts or you feel suddenly worse.

After the first shock, people return to the same question: what causes low red and white blood cells? In many cases, the answer shows up once the CBC is repeated, meds are reviewed, and a few targeted labs are added.