Pre-operative lab value tutorial

An important part of the preoperative evaluation is knowing when it is appropriate to order certain labs and how to evaluate their values. This can be, at times, very tricky even for anesthesia providers that have been practicing for some time. As a brand new student, seeing and evaluating all of these numbers can be very overwhelming. This tutorial is meant to help make evaluating labs a less daunting task. If nothing else, you should leave this page with the ability to recognize when lab values are outside of their normal range. Being able to know what to do, if anything at all, is another story and will eventually come with time. This is a starting point. If you see lab values that are out of the norm, use it as an opportunity to question why this is the case. Start a discussion with your instructor any chance you get, and it will start to make sense sooner.

Obviously we can’t order every available lab value for every single patient. For some people and surgeries, it would just be a waste. As with many things in anesthesia, it is important to weigh out the risk and cost versus the benefits when deciding what labs to order. Often times, you will not be the person ordering the labs, but it is something important to start thinking about; especially when it comes to acknowledging when an important value has not yet been determined for a patient. Ultimately lab values can aid you in confirming or excluding the presence of a disease, reduce risk, limit patient morbidity and mortality during surgery, and reduce costs. In addition to this, they can also be costly and time consuming to evaluate. You will have to use your best judgment to determine when you think they are needed.

BMP – Basic metabolic panel (AKA Chem-7)

Can potentially clue you in to information about fluid and electrolyte balances, kidney function, blood-glucose levels, and responses to various medications.

When might it be a good idea to order?

• Underlying disease: HTN, Obesity, Heart failure, Kidney disease, liver disease, diabetes, CNS disease, endocrine disorders.
• Using certain meds: diuretics, ACE inhibitors & ARBs, NSAIDs, Steroids, Digoxin, Aminoglycosides.

Normal ranges of values:

• Na – 135-145 mEq/L
• K – 3.5-5.1 mEq/L
• Cl – 95-109 mEq
• HCO3 – 22-26mmol/L
• BUN -7-22mg/dL
• Cr – 0.5-1.4 mg/dL
• Glucose(fasting) – 70-100mg/dL

A few values to focus on:

• Cr: Creatinine – Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body. Men generally have more muscle, so their levels will be elevated. It is excreted unchanged by the kidneys, so provides a good evaluation of kidney function. It is inversely related to GFR. In geriatric patients, muscle mass decreases while kidney function is decreased, leaded to Cr levels remaining relatively unchanged in patients with relatively good kidney function.  

• BUN – Blood Urea Nitrogen – Produced in the liver. It is directly related to protein catabolism and inversely related to GFR. Not always a reliable indicator of GFR, because protein catabolism may be impaired.

• Bun:Cr ratio: Normally = 10:1.      >15:1 can be a sign of volume depletion in patients.  

• K – Potassium – Generally speaking, elevated potassium levels are more of a concern than low levels. Low levels will rarely cause any issued or give reason for delayed surgery, unless they reach critical levels close to 2.0. In this case, they can be a cause for rhabdomyolysis. May be low in patients taking diuretics. Elevated levels on the other hand are more a cause for concern. You will often see elevated levels in end stage renal disease patients, patients taking potassium sparking diuretics, and patients taking ACE inhibitors or ARBS. Large increase can cause progressive EKG changes and eventually lead to cardiac arrest if not properly treated. Drugs such as succinylcholine can lead to increased levels of K, so be careful when using.

• Glucose – Maintaining blood glucose close to the patient’s baseline can play an important role in preventing surgical site infection. Low glucose levels can make patients lethargic and lead to delayed emergence from anesthesia.

Coagulation Studies

Determine the clotting tendency of blood.

When might it be appropriate to order?

• Known coagulation disorders, age >75, kidney or liver disease, diabetic, major procedure with lots of anticipated blood loss, planned epidural/spinal placement, taking anticoagulant therapy, pregnancy, trauma, sepsis.

Normal range of values:

• PT – 10-12 secs
• PTT – 25-35 secs
• INR – 0.8-1.2 secs

What these values can tell you:

• PT – Prothrombin time – Prolonged when there are issues with the extrinsic clotting pathway. Often see elevated times in patients with vitamin K deficiency, liver disease, and on warfarin therapy.

• PTT – Activiated Partial Thromboplasin Time –Prolonged when a patient has issues with the intrinsic pathway. Often see elevated times in patients receiving Heparin, hemophiliacs, and Von Willebrand’s pateints.

• INR – International Normalized ratio – Basically the same as PT. It is a standardized PT, designed to account for differenced in thromboplastin. Target for patients on Warfarin is generally 2.0-3.0.

CBC – Complete Blood Count

Provides information about the cells inside the patient’s blood.

When might it be a good idea to order?

• Age >75, major procedure with lots of anticipated blood loss, chronic kidney or liver disease, anticoagulant use, bleeding disorders, clinical signs of anemia, trauma, infection, pregnancy.

Normal range of values:

• WBC – 4.0-10.5
• Hemoglobin – Men: 14-18g/dL; Women: 12-16g/dL
• Hematocrit – Men: 40.7 – 50.3%; Women: 36.1 – 44.3%
• Platelets- 150 – 440
• What these values can tell you:
• WBC – Elevated levels can be a sign of infection or sepsis.

What these values can tell you:

• Hemoglobin – Low values can tell you when a patient is anemic. In these situations, they may have recently been bleeding, are currently bleeding, or may have impaired kidney function. Elevated levels usually mean the patient is dry and may need fluid resuscitation with IV crystalloids.  

• Hematocrit = about 3xHbg. Basically clues you in to the same things as listed above.

• Platelets – The most important value in evaluating primary hemostasis. Below 150 is considered thrombocytopenia. Lower values are associated with increased intraoperative bleeding.

I hope this provides a good starting point and reference. Don’t be afraid to come back and review this material over time. It will continue to be useful throughout your time as a student and a practitioner.