Which Blood Tests Are Done Before Surgery, and Why

Undergoing surgery involves more than just the procedure itself. Your medical team will order a panel of blood tests before surgery to make sure you’re healthy enough to go under anesthesia, to reduce risks, and to prepare for any complications that might arise. Below, we explain blood tests before surgery what they check, why each test matters, and how the results guide your care.

Remember: if you have any serious or life-threatening symptoms, speak to a doctor right away. You may also consider doing a free, online symptom check, using the doctor approved Ubie Symptom Checker.

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1. Complete Blood Count (CBC)

What it checks:

• Red blood cells (RBCs) and hemoglobin: oxygen delivery to tissues
• White blood cells (WBCs): signs of infection or immune issues
• Platelets: blood clotting ability

Why it matters:

• Anemia (low hemoglobin) can increase fatigue and slow healing.
• A high or low WBC count may signal an infection that needs treatment before surgery.
• Low platelets or abnormal platelet function raises the risk of bleeding during and after the operation.

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2. Electrolytes and Minerals

What it checks:

• Sodium, potassium, chloride, bicarbonate
• Calcium, magnesium, phosphorus (sometimes included)

Why it matters:

• Electrolyte imbalances can affect heart rhythm and muscle function.
• Correcting low potassium or sodium levels beforehand lowers the chance of dangerous heart rhythms under anesthesia.
• Proper mineral balance supports normal nerve and muscle activity during recovery.

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3. Kidney Function Tests

What it checks:

• Blood urea nitrogen (BUN)
• Creatinine
• Estimated glomerular filtration rate (eGFR)

Why it matters:

• Kidneys clear many anesthesia drugs and waste products.
• Poor kidney function can lead to drug buildup and toxicity.
• Identifying reduced kidney function allows your team to adjust medication doses or fluids.

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4. Liver Function Tests (LFTs)

What it checks:

• Alanine aminotransferase (ALT)
• Aspartate aminotransferase (AST)
• Alkaline phosphatase (ALP)
• Bilirubin

Why it matters:

• The liver processes anesthesia and pain medications.
• Elevated LFTs can signal liver disease, which may require special drug dosing or postponing surgery.
• Normal liver function helps ensure medications are broken down safely.

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5. Coagulation Profile

What it checks:

• Prothrombin time (PT) and international normalized ratio (INR)
• Activated partial thromboplastin time (aPTT)

Why it matters:

• These tests measure how quickly your blood clots.
• A prolonged PT/INR or aPTT indicates a higher bleeding risk.
• If needed, doctors can correct clotting factors before the operation to minimize blood loss.

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6. Blood Type and Crossmatch (Type & Screen)

What it checks:

• ABO blood group (A, B, AB, O)
• Rh factor (positive or negative)
• Antibody screen and crossmatch (if transfusion is likely)

Why it matters:

• Ensures compatible blood is available in case a transfusion is needed.
• Speeds up the process during unexpected bleeding.
• Reduces the risk of transfusion reactions by matching donor blood to your type.

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7. Blood Glucose (Sugar)

What it checks:

• Fasting blood glucose or A1C (long-term glucose control)

Why it matters:

• High blood sugar (hyperglycemia) can impair wound healing and increase infection risk.
• Low blood sugar (hypoglycemia) can cause dizziness, confusion, and complications under anesthesia.
• If you have diabetes, knowing your baseline helps anesthesiologists adjust insulin or other medications.

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8. Additional and Specialized Tests

Depending on your age, overall health, and type of surgery, your doctor may order extra tests to further reduce risks:

• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR): look for inflammation
• Cardiac markers (troponin) and B-type natriuretic peptide (BNP): heart stress or damage
• Thyroid function tests (TSH, T3, T4): especially if you have known thyroid disease
• Pregnancy test (hCG) for women of childbearing age
• HIV, hepatitis B or C screening: if risk factors are present
• Vitamin levels (e.g., B12, D): in certain nutritional or bone procedures

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How Results Guide Your Care

1. Rescheduling or further evaluation
   • If a serious issue is found (eg, uncontrolled infection, severe anemia, critical electrolyte imbalance), your surgery may be postponed until the problem is treated.
2. Medication adjustments
   • Blood thinners, diabetes drugs, or heart medications may be temporarily stopped, changed, or monitored more closely.
3. Anesthesia planning
   • Anesthesiologists tailor drug choice and dosing based on liver/kidney function, heart health, and your clotting ability.
4. Blood product preparation
   • Ensuring the right type and amount of blood products are ready in the operating room.
5. Postoperative monitoring
   • Abnormal preoperative results flag the need for closer observation in recovery, such as extra vitals checks or lab tests.

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Tips for a Smooth Testing Experience

• Follow fasting instructions. Many tests require you to skip food and drink (except water) for 8–12 hours beforehand.
• Bring a list of medications. Include prescription, over-the-counter, and supplements.
• Stay hydrated. Unless told otherwise, drinking water helps make blood draws easier.
• Wear a short-sleeve shirt or loose sleeve. This gives the phlebotomist easy access to your arm.
• Ask questions. Understanding why each test is done can help ease any concerns.

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Final Thoughts

Preoperative blood tests play a crucial role in making surgery as safe as possible. They let your care team:

• Detect hidden issues
• Optimize medication plans
• Prepare for emergency needs
• Reduce complications during and after surgery

If you’re unsure about any symptom or test result, speak to your doctor. For a quick, free assessment of non-urgent symptoms, you might consider doing a free, online symptom check, using the doctor approved Ubie Symptom Checker. Always reach out to a medical professional about anything that could be life threatening or serious.

By understanding blood tests before surgery what they check, you’ll feel better prepared and more confident heading into the operating room. Good preparation is a key part of a successful recovery—and your medical team is there to guide you every step of the way.