While you’re going to ultimately learn a lot about labs throughout nursing school, you’re not going to learn them all at once. So, I’ve compiled eight key labs to know thoroughly before your first clinical day. These are hemoglobin, hematocrit, WBCs, platelets, sodium, potassium, creatinine, and glucose. Are these the only labs you need to understand? Of course not, but it will get you off to a great start!

Hemoglobin

Hemoglobin is the protein molecule in red blood cells that carries oxygen and carbon dioxide throughout the body. It picks up oxygen at the alveolar level and exchanges it for carbon dioxide in the body’s tissues. Hemoglobin is a component of the complete blood count (CBC) and is a reflection of the number of RBCs in the blood.

Though you probably think of hemoglobin as an indicator in anemia, which is the condition in which hemoglobin is low, it can also be elevated in some cases. Causes for increased hemoglobin include polycythemia vera, COPD, certain cancers, pulmonary fibrosis, smoking, dehydration, living at high altitude, congenital heart disease, and taking medications that increase RBC production such as erythropoietin-stimulating agents.

Notice that dehydration was mentioned as a possible cause for elevated hemoglobin levels. This is because hemoglobin levels are relative to plasma volume. So, if an individual is dehydrated, the blood becomes more concentrated and hemoglobin levels are higher.  If the patient has increased plasma volume, the blood is less concentrated and hemoglobin levels can be lower, such as in pregnancy. So, when you are looking at your hemoglobin values, take the patient’s fluid volume status into account.

Conditions associated with low hemoglobin levels include anemia, cirrhosis, pregnancy (due to increased plasma volume), chronic kidney disease, bone marrow failure, splenomegaly, GI bleeds, and hemolytic reactions which can occur with blood transfusion.

Speaking of blood transfusions, hemoglobin is generally the lab value utilized to determine if a patient requires a blood transfusion. Though a normal hemoglobin level is between 12 to 18 g/dL (with some variability for gender), the criteria for a blood transfusion is typically a hemoglobin level less than 7 or 8 g/dL in the acute care setting. However, this threshold may be higher in a patient who is actively bleeding and, of course, is dependent upon the physician and the patient’s symptoms. Each unit of blood should raise the hemoglobin level by approximately 1 g/dL. 

A question that comes up often is when the hemoglobin level should be reassessed after a transfusion and you will likely hear different answers. A 2020 study showed that there was no significant difference in the hemoglobin level when tested one hour, four hours, and 24 hours after the transfusion. So, if you are drawing labs on a patient who has just had a blood transfusion and want a true reflection of the hemoglobin level, you can get a reliable result after just one hour.

So what might you notice about a patient with elevated or low hemoglobin levels? A patient with elevated hemoglobin levels may have headache, hypertension, blurred vision, and fatigue. If the hemoglobin is especially high the blood may be too viscous to travel effectively through tiny capillaries and the patient can suffer ischemic events including stroke.

A patient with lower hemoglobin levels may experience fatigue, weak pulses, cool extremities, pale skin and if severe, shortness of breath, lightheadedness, and even chest pain. If your patient is symptomatic with low hemoglobin levels, they should be kept on bed rest until the MD determines it is safe for them to increase activity. Usually this is after hemoglobin levels increase, either on their own or with a blood transfusion.

Hematocrit

Hematocrit is very similar to hemoglobin, but it’s important to understand what the value represents. Hematocrit is a measure of the percentage of total blood volume that is made up of red blood cells. Hematocrit is generally three times the hemoglobin and can also be affected by plasma volume levels. The hematocrit is expected to increase by 3% with each unit of packed red blood cells.

PRO TIP! Hematocrit is often measured alongside hemoglobin. If you hear someone refer to the “H & H” they are referring to the hemoglobin and hematocrit. 

WBCs

Another important component of the complete blood count is the white blood cell (WBC) count). This is the key lab you’ll assess as you monitor your patient with suspected or confirmed infection. A normal WBC count is about 5,000 to 10,000 in an adult. 

An increased total WBC count is called leukocytosis, and this is generally a sign of infection. However, other causes for leukocytosis can include inflammation, trauma, stress, smoking, and certain medications such as corticosteroids and lithium. White blood cells are also increased during labor and in later stages of pregnancy, so an elevated level does not necessarily mean the individual has an infection. Other rare but possible causes of leukocytosis include certain types of cancer such as acute myeloid leukemia and chronic lymphocytic leukemia.

Without getting into too much detail, a secondary test called the differential looks at different types of WBCs – neutrophils, lymphocytes, monocytes, eosinophils, and basophils. One important thing to know is that an increase in a specific type of neutrophils called “bands” generally indicates an acute or worsening bacterial infection.

White blood cells can also be decreased in infection if the body isn’t able to mount a sufficient immune response or if the body’s WBCs are depleted by overwhelming infection. Other conditions associated with leukopenia include bone marrow failure, autoimmune disease, malnutrition, and certain medications such as immunosuppressants, clozapine (an antipsychotic), hydroxychloroquine (used to treat systemic lupus erythematosus), lamotrigine (an antiseizure drug), diuretics and chemotherapeutic agents.

If your patient has a very low WBC count and specifically a very low neutrophil count, they will likely be placed on neutropenic precautions and be at very high risk for infection. You will need to thoroughly wash your hands before entering the patient’s room and wear full PPE. Many times the room will have positive pressure airflow to help keep airborne pathogens out of the patient’s room. Regardless of whether or not your patient has an airflow isolation room, you must keep the door closed at all times. Before leaving the room make sure the patient has easy access to their call light should they require assistance as you may not hear them through a closed door.

Platelets

One more key component of the complete blood count is the patient’s platelet level. A low platelet level is thrombocytopenia and an elevated platelet level is thrombocytosis. Both can be detrimental for your patient, though thrombocytopenia is far more common. 

A low platelet level is generally below 100,000 and the greatest risk for the patient is bleeding. In fact, spontaneous hemorrhage can occur when platelet levels are below 20,000. Causes of thrombocytopenia are numerous and include chemotherapy, leukemia, bone marrow failure, DIC, viral infections, lupus, and nutritional deficiencies. In addition, a common cause of thrombocytopenia is a condition called immune thrombocytopenic purpura (ITP), which is an autoimmune condition resulting from the presence of antiplatelet autoantibodies.

An elevated platelet count occurs in conditions such as polycythemia vera, malignancies, splenectomy, hemolytic anemia, inflammatory disorders such as rheumatoid arthritis, and iron deficiency anemia. Thrombocytosis can also occur with blood losses as the body tries to stop the bleeding. 

Sodium

There are multiple mechanisms in the body that affect serum sodium levels and these are closely tied with fluid balance. For example, aldosterone and antidiuretic hormone cause the body to hold on to sodium, which therefore causes the body to hold on to water. The body may produce more or less of these hormones in efforts to maintain homeostasis.

Because sodium is most abundant outside of the cell, normal plasma levels are quite high when compared to electrolytes that mostly live inside the cell such as potassium. A normal serum sodium level is between 135 and 145 mEq/L. An elevated serum sodium level is called hypernatremia while a low level is called hyponatremia. In the clinical setting, hyponatremia is one of the most common electrolyte abnormalities you will see.

The reason hyponatremia is concerning is that it is closely associated with cerebral edema and neurological complications. And this is because of the way fluid shifts in the body when sodium levels are low. When the concentration of sodium is low in the serum, the serum is essentially hypotonic. The osmotic gradient causes fluid to shift out of the intravascular space and into brain cells. This causes cerebral edema and can lead to serious neurological deterioration, especially if the shift is drastic and occurs quickly. 

Note that hyponatremia can also be chronic. These individuals tend to have less drastic neurological compromise and may even be asymptomatic, especially if the hyponatremia is mild. 

Hyponatremia can be due to a variety of factors including excess water consumption, decreased sodium intake, Addison’s disease (which causes increased losses due to low aldosterone levels), losses through the GI tract (diarrhea, vomiting or nasogastric suctioning), hyperglycemia, and third-spacing of fluids.

Mild hyponatremia is often treated by restricting free water intake and letting the body achieve homeostasis. If the hyponatremia is severe or the patient is showing signs of neurological compromise, hypertonic saline may be utilized. Note this is a high risk medication and sodium levels must be corrected very slowly and monitored carefully so as to avoid sudden fluid shifts and a serious complication called central pontine myelinolysis or locked-in syndrome.

Hypernatremia is most often due to water losses which can occur due to diabetes insipidus, extensive burns, and simply because of decreased PO fluid intake. Another potential cause is increased sodium reabsorption which occurs in conditions such as hyperaldosteronism and Cushing’s syndrome. Hypernatremia can also be due to excessive sodium intake either from IV fluids or dietary sources. The most significant complication of hypernatremia is subdural or subarachnoid hemorrhage due to vascular rupture.

Hypernatremia is also corrected slowly since rapidly decreasing sodium levels can cause detrimental fluid shifts, cerebral edema and seizures. Treatment involves addressing the underlying cause and may also include IV fluids or increased oral intake of water, depending on the severity.

Potassium

The main reason you want to check your patient’s potassium level is because of the role it plays in cardiac electrophysiology. High or low potassium levels can lead to serious cardiac dysrhythmias and even cardiac arrest. 

Potassium is the main cation inside the cell, so serum levels are actually quite small. A normal serum potassium level is approximately 3.5 to 5.0 mEq/L and even small changes can have a big impact. Since potassium is secreted by the kidneys and resorption does not occur, levels can drop dramatically in individuals who are not taking in dietary potassium or receiving supplementation through tablets or IV fluids.

Patients at highest risk for low potassium levels are those taking loop diuretics, such as furosemide. These patients often require potassium replacement, which can be PO with tablets or an oral liquid, or it can be IV. It is absolutely imperative to understand that IV potassium is never given as an IV push as this can cause cardiac arrest. Instead, IV potassium is administered in diluted form at a slow rate of 10 mEq/hour in most cases.  

If your patient is on potassium replacement protocol, make note of when their next potassium level should be checked. If the potassium was administered PO, this is generally four to six hours later. If it was administered IV, it is often checked one hour after the total dose has fully infused. 

Signs of hypokalemia are generally related to muscle contraction so you may notice weakness, gastric ileus, and depressed cardiac function which often manifests initially as PVCs.

Hyperkalemia is also a concern and the sign that always shows up on exams is that hyperkalemia can cause tall, peaked T waves on the EKG. While hypokalemia causes weakness and gastric ileus, hyperkalemia can cause irritability, diarrhea and vomiting. 

Hyperkalemia can be caused by multiple factors including excessive IV intake, renal failure, transfusion of hemolyzed blood, hemolysis, and acidosis. Another common cause is the patient who is taking a potassium-sparing diuretic such as spironolactone. If they are consuming potassium-rich foods or receiving a supplement, they are at much higher risk for elevated potassium levels. 

How hyperkalemia is treated depends on its severity. If mild and the patient is not symptomatic, treatment may include the administration of kayexalate which binds potassium in the GI tract to be excreted with the stool. If faster treatment is necessary, insulin given IV along with dextrose can rapidly shift excess potassium into the cell. This is because insulin unlocks the cell and potassium enters along with the dextrose. You’ll also notice that calcium is typically also administered IV at this time. The calcium does not affect potassium levels and is instead given to make the cardiac cells less likely to suffer from the cardiotoxic effects of hyperkalemia.

Another medication that causes potassium to shift into the cell is albuterol, which has shown to be just as effective as insulin with a longer duration of action. In critical cases, emergency dialysis may be necessary.

Creatinine

An important lab value that relates to kidney function is creatinine, which is a component of the basic metabolic panel (BMP) or the complete metabolic panel (CMP). Creatinine is a waste product that comes from the breakdown of muscle tissue and protein digestion. 

Since it is entirely excreted by the kidneys, it serves as a key indicator of kidney function. When the kidneys are functioning well, creatinine is removed adequately from the body. When creatinine levels are elevated, this is an indicator of impaired renal function (and possibly even renal failure). Reasons for an elevated creatinine include chronic kidney disease, acute conditions such as glomerulonephritis, urinary obstruction, nephrotoxic medications (such as cephalosporins and ACE inhibitors), and simple dehydration. As a general rule, a doubling of the creatinine level represents a 50% reduction in glomerular filtration rate.

It’s important to understand how your patient’s kidney function affects other aspects of their care. For example, a patient with markedly elevated creatinine may be at risk for electrolyte imbalances such as hyperkalemia (which can cause cardiac dysrhythmias). Additionally, many medications require reduced dosing when creatinine levels are elevated, such as enoxaparin. And, some medications, such as metformin, may be avoided altogether when serum creatinine levels are increased. Additionally, a patient with acute renal failure is at risk for fluid volume overload, pulmonary edema, and even seizures.

Glucose

Another key lab test to understand is glucose, which you can find on the BMP, CMP and through a POC test using a fingerstick. Patients are monitored in the clinical setting for hypoglycemia and hyperglycemia, even when they do not have diabetes. 

Hypoglycemia can occur for multiple factors, the main ones being decreased nutritional intake and medications. In addition to medications that directly reduce blood glucose, such as insulin, other medications that can cause hypoglycemia include beta blockers, indomethacin, and antibiotics, among others. Patients with hypoglycemia are at risk for devastating neurological complications, so if the levels cannot be improved with increased PO intake, then IV dextrose is typically necessary.

Hyperglycemia occurs for reasons beyond diabetes as well. One of the most common reasons for elevated blood glucose in the clinical setting is stress, which can be related to trauma, surgery, inflammation and infection. In fact, an elevated blood glucose level is one of the supporting factors for a diagnosis of sepsis. Other causes for elevated blood glucose include excess intake (from PO sources or IV fluids) and medications such as corticosteroids and tricyclic antidepressants.

Prolonged hyperglycemia (such as with uncontrolled diabetes or long-term corticosteroid use) puts the patient at risk for infection, poor wound healing, and poor skin integrity. Additionally, the evidence shows that hyperglycemia is associated with poorer outcomes, increased length of stay, and increased morbidity and mortality among hospitalized patients. It is therefore very important to ensure the MD is notified when blood glucose testing reveals unexpected hyperglycemia so it can be managed effectively in the clinical setting.


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