Learning Objectives
By the end of this section, you will be able to:
- Describe the chemical, physical, and microscopic characteristics of blood
- Verbalize the steps in collecting a blood specimen
- Interpret the results of a blood sample analysis
Nurses often obtain blood samples for testing, which is a common and essential component of patient care and diagnostics. As healthcare professionals, nurses understand that the quality and accuracy of these specimens are critical, as they hold the key to uncovering invaluable insights into a patient’s health. Understanding the chemical, physical, and microscopic characteristics of blood provides a foundation for recognizing abnormalities. As a nurse, you will be responsible for collecting blood specimens, ensuring accurate and contamination-free sampling. Knowing the normal characteristics of blood provides nurses with a baseline to be able to interpret the results of blood testing, enabling the nurse to make informed clinical decisions for their patients.
Blood Specimen Overview
Blood is a term used to describe the liquid that moves through the vessels and includes plasma (the liquid portion, which contains water, proteins, salts, lipids, and glucose), the cells (red and white cells), and cell fragments called platelets. Blood plasma is actually the dominant component of blood and contains water, proteins, electrolytes, lipids, and glucose. Blood is 20 percent of a person’s extracellular fluid and 8 percent of their weight. The four main components of blood are plasma, red blood cells, white blood cells, and platelets.
Link to Learning
More details about the composition and function of blood are presented in this video.
Purpose
Blood collection serves multiple purposes in health care. It is a fundamental diagnostic tool used to gather valuable information about a person’s health and medical conditions. Blood testing is used for a variety of things, including the following:
- diagnosing certain diseases and conditions
- monitoring a chronic disease or condition
- evaluating effectiveness of a treatment
- evaluating organ function
- diagnosing clotting or bleeding disorders
- evaluating the immune system
Types of Blood Collection
Nurses collect blood samples from patients using several methods, including venipuncture, capillary blood sampling, and blood draws from venous access devices. Blood may also be drawn from arteries by specially trained professionals for certain laboratory testing.
Venous
The process of venipuncture involves introducing a needle into a patient’s vein to collect a blood sample or insert an intravenous (IV) catheter. Blood sampling with venipuncture may be initiated by nurses, phlebotomists, or other trained personnel. Venipuncture for collection of a blood sample is an important part of data collection to assess a patient’s health status. It is commonly performed to examine hematologic and immune issues such as the body’s oxygen-carrying capacity, infection, and clotting function. It is also useful for assessing metabolic and nutrition issues such as electrolyte status and kidney functioning. Venous blood is relatively easy to collect and is preferred because veins are superficial and have less nerves associated with them (Srikanth & Lotfollahzadeh, 2023).
Arterial
In arterial blood sampling, blood is obtained via puncture into an artery by specially trained registered nurses and other healthcare personnel, such as respiratory therapists, physicians, nurse practitioners, and physician assistants. Arterial blood collection is most commonly performed to assess the body’s acid-base balance in a diagnostic test called an arterial blood gas. The most common access site for arterial blood sampling is the radial artery. Arterial blood tests are known to be more painful for the patient than venipuncture and have a higher risk of complications such as bleeding and arterial occlusion with subsequent ischemia to the area distal to the puncture.
Capillary
In capillary blood collection, blood is obtained from capillaries just beneath the skin’s surface, often from the fingertips or heel (in infants). A lancet device is used to puncture the skin and collect a small drop of blood on a capillary tube, filter paper, or test strip. Capillary blood is frequently used for point-of-care (POC) testing, such as glucose monitoring for diabetes, as well as for newborn screenings. Capillary blood is ideal for POC and rapid tests because it is readily accessible; however, it may not provide the same depth of analysis as venous or arterial blood.
Procedural Steps for Blood Collection
For each type of blood collection, there is an associated procedure for collecting the blood sample. It is imperative for nurses to not only know how to demonstrate each type of blood collection method but also be skilled in selecting an appropriate site for collecting the blood sample. Understanding the factors that guide the choice of site selection is paramount to ensure the procedure’s accuracy and the patient’s comfort. This section equips you with the skills and knowledge to perform venous, arterial, central line, and capillary blood collections.
Venous Blood Collection
Performing a venipuncture involves selecting an appropriate site and placing a tourniquet (tightly tied band applied around a limb to temporarily constrict or restrict blood flow) several inches above the selected venipuncture site to slow the blood flow in the veins, making the veins more visible and accessible. Ensure the tourniquet is tight enough to engorge the veins without causing pain but not so tight as to impede arterial blood flow.
Clean the venipuncture site with an antiseptic agent and allow the site to completely air dry to avoid contaminating the sample. After informing the patient you are about to perform the venipuncture, puncture the vein quickly and smoothly. Once blood begins to flow, adjust the angle of the needle to be nearly parallel with the skin. Be cautious not to puncture through the vein. Release the tourniquet as soon as blood starts to flow, as prolonged use may alter blood chemistry. Fill the required amount of blood into the vacutainer(s) or syringe.
Once the required volume is obtained, withdraw the needle smoothly and gently. Apply pressure and a sterile adhesive bandage or gauze to the venipuncture site to stop any bleeding. Dispose of equipment in the appropriate receptacles, label the blood collection tube appropriately, and place the sample in a biohazard bag to send to the laboratory for analysis.
Link to Learning
Some helpful tips for drawing blood are presented in this video.
Clinical Safety and Procedures (QSEN)
QSEN Competency: Performing a Venipuncture
See the competency checklist for Performing a Venipuncture. You can find the checklists on the Student resources tab of your book page on openstax.org.
Vein Selection
Selecting an appropriate vein for venous blood sample collection is a critical step in ensuring a successful and comfortable procedure. Assess the patient’s medical history and condition to check for any known issues or complications related to venous access, such as past difficulties in blood collection or potential vein-related conditions. Visually inspect the patient’s arms, hands, and wrists to identify visible and palpable veins. Consider using the nondominant arm, but the choice may depend on patient preference or previous issues with blood collection. Look for veins that are straight, visible, and easily palpable, as these are typically easier to puncture and yield blood more readily. The antecubital fossa (the bend of the elbow) is a common site for venipuncture due to its accessibility and typically larger veins. Consider alternative sites, such as the back of the hand, the wrist, or the forearm, especially if the antecubital veins are not suitable.
Vein size and depth can vary significantly from person to person. Choose a vein that is of appropriate size for the required blood volume and is not too deep so that the site can be punctured comfortably.
Veins that feel like they “bounce back” when gently pressed are often more suitable for venipuncture. Avoid selecting veins that show signs of damage, scarring, or thrombosis (clotting). Damaged veins may be more painful and less suitable for blood collection.
Communicate with the patient about their comfort and preferences regarding vein selection. Some individuals may have preferences based on past experiences or discomfort in certain areas. Be sensitive to any concerns the patient may have and try to accommodate their preferences, if possible. If the selected veins do not seem prominent enough, a warm compress may be applied to the area for a few minutes to help dilate the veins and make them more accessible.
Collection Devices
Venous blood collection devices are tools and equipment used by healthcare professionals to collect blood samples safely and efficiently from a patient’s vein. When collecting venous samples, you will need a needle, vacutainer, and vacutainer tubes. There are several different types of needles to pick from, including a vacutainer needle (a needle with a built-in vacutainer adapter), a butterfly needle (winged infusion set with a vacutainer attached), and a multiple-draw needle (needle that connects to a separate vacutainer prior to the collection tubes being attached) (Figure 10.8). Vacutainer needles come in various sizes, including 21-, 22-, and 23-gauge needles, and are attached to vacutainer tubes to draw blood directly into the tubes. Butterfly needles (winged infusion sets) are smaller and more flexible than standard needles and are often used for patients with difficult-to-access veins or those who require frequent blood draws. Multiple-draw needles allow multiple blood tubes to be collected sequentially without needing to remove the needle from the vein. This is a built-in safety feature to help avoid needlesticks.
A sterile, color-coded tube used to collect and transport a blood sample is called a vacutainer tube (Figure 10.9). This type of tube possesses a partial vacuum due to a rubber stopper, resulting in negative air pressure within the tube that is lower than the surrounding environment. Once the needle is inserted into the vein, the nurse places the vacutainer tube into the vacutainer holder, piercing the stopper and enabling the blood to flow into the tube.
A vacutainer adapter is a plastic device that holds the vacutainer tube in place and connects to the needle for blood collection. It provides stability and ensures that the tube fills correctly. The main component of the adapter is a plastic body with a grip handle. The adapter’s design allows for easy manipulation by healthcare professionals during the venipuncture procedure. The adapter typically features a Luer-lock connection where a needle can be securely attached.
Steps for Central Line Collection
A central venous access device (CVAD) is a long, thin tube inserted into a large vein, typically in the chest or neck. CVADs may have a single lumen (opening), double lumen, or multiple lumens that exit at various places along the central catheter (Figure 10.10). Each lumen has a different use based on its exit point (Table 10.8). A single lumen device consists of one lumen and is typically used in procedures requiring a straightforward fluid pathway. Double lumens incorporate two lumens, allowing for two-way fluid flow or simultaneous administration of different substances. Triple lumen devices, with three lumens, provide even greater versatility, enabling healthcare providers to perform multiple tasks concurrently, such as administering fluids, administering medications, and monitoring central venous pressure. The choice among single, double, or triple lumens depends on the complexity of medical procedures and the diverse functions required.
| Lumens | Proximal Lumen | Middle Lumen | Distal Lumen |
|---|---|---|---|
| Size | 18 gauge | 18 gauge | 16 gauge |
| Uses | Fluids Total parenteral nutrition/lipids Medications |
Medications | Blood draw Blood administration Central venous pressure monitoring |
Obtaining a blood sample from a CVAD is a responsibility of the registered nurse. Multiple venipunctures frequently occur in acute care settings due to the severity of the medical condition in a patient who requires a CVAD. The main advantage of using a CVAD for frequent blood sampling is decreased pain and anxiety compared to the experience of multiple peripheral venipunctures. However, accessing CVADs also has potential risks associated with infection, occlusion, and improper sample taking, resulting in inaccurate test results. Following evidence-based infection prevention practices, limiting the frequency of blood sampling, and following aseptic no touch technique (ANTT) guidelines help reduce the risk of infection. Note that current guidelines recommend not to use CVADs infusing parental nutrition for blood sampling because manipulation may increase the risk for central line-associated bloodstream infection (Infusion Nurses Society, 2024). During the blood sampling procedure, if any signs or symptoms occur indicating an air embolism, place the patient on the left side in Trendelenburg or left lateral decubitus position, call the rapid response team, and notify the provider. A summary of the key points of blood sampling from a CVAD is outlined in Table 10.9.
| Steps | Rationale |
|---|---|
| Stop the infusion of fluids and medications into the catheter’s lumens. | Stopping the infusion of fluids or medications prevents these substances from interfering with the blood sample. Current guidelines do not specify a standard length of time for stopping the infusion, but the length of time is associated with the internal volume of the specific CVAD (Infusion Nurses Society, 2024). |
| Choose the appropriate CVAD lumen for obtaining samples based on the largest lumen or the configuration of the lumen exit sites. | Blood draw requires a large lumen. For catheters with a staggered lumen exit at the tip, the sample should be drawn from the lumen exiting at the point farthest away from the heart and above other lumen exits used for infusion (Infusion Nurses Society, 2024). Follow CVAD manufacturer’s instructions for these decisions. |
| Vigorously scrub the needles’ connector for at least fifteen seconds with antiseptic scrub and let it dry completely (The Joint Commission, 2023). | Scrubbing prevents microorganism contamination, and drying prevents contamination by substances. |
| Attach a prefilled 10 mL syringe of preservative-free normal saline to the needleless connector using ANTT. Unclamp the catheter and thoroughly flush the lumen with 10 to 20 mL of preservative-free 0.9 percent normal saline (Infusion Nurses Society, 2024). Aspirate slowly for blood, noting the characteristics of the blood. | A 10 mL syringe generates lower pressure within the catheter and prevents lumen rupture and/or occlusion. |
| Clear the dead space by using the push-pull method or discarding the aspirated blood according to facility policy. The discard method requires initial aspiration of 2 to 25 mL of blood (per internal volume of the CVAD, saline flushing prior to drawing the discard volume, and specific laboratory tests needed), and then discarding the syringe before performing the blood sampling (Infusion Nurses Society, 2024). The push-pull method utilizes the same syringe used when aspirating to test patency of the catheter. With the syringe still attached, 4 to 6 mL of blood is aspirated and then pushed back into the catheter. This aspiration and reinfusion sequence is repeated for four cycles. The blood and syringe are then discarded (McBride et al., 2018). | Either method clears the CVAD catheter’s dead space volume and removes any of the blood that becomes diluted with the flush solution. Performing the push-pull method for four cycles allows for an accurate blood sample and also reduces phlebotomy-associated blood loss, particularly when obtaining multiple blood samples. For the discard method, coagulation studies require the largest discard volume to produce accurate results, but this volume of discarded blood can lead to hospital-acquired anemia. |
| After obtaining the blood sample, thoroughly flush the CVAD lumen with 10 to 20 mL of preservative-free 0.9 percent normal saline (Infusion Nurses Society, 2024). | Flushing thoroughly prevents occlusion. |
Clinical Safety and Procedures (QSEN)
QSEN Competency: Collecting a Central Line Specimen
See the competency checklist for Collecting a Central Line Specimen. You can find the checklists on the Student resources tab of your book page on openstax.org.
Arterial Blood Collection
Drawing an arterial blood sample involves collecting blood from an artery (Figure 10.11), typically the radial artery in the wrist or the femoral artery in the groin.
Performing arterial blood sampling involves several key steps to ensure accuracy and safety. One crucial aspect is assessing collateral blood flow, often done using the modified Allen test to determine the patency of the ulnar artery. This test assesses the adequacy of collateral circulation in the hand before arterial puncture. After confirming sufficient collateral flow, the procedure involves proper hand positioning, sterilization, and selecting an appropriate arterial site, commonly the radial artery. The use of aseptic techniques, ensuring patient comfort, and promptly analyzing the blood sample are essential in the process. Postprocedure, adequate pressure and dressing application to the puncture site help minimize bleeding and promote optimal healing. Careful attention to these steps is vital for accurate arterial blood sampling and ensuring patient safety during the procedure.
Site Selection
Site selection for arterial blood collection is a crucial aspect of the procedure, as it affects the ease of sample collection, patient comfort, and quality of the obtained sample. The radial artery is the most commonly used site for arterial blood collection due to its accessibility and relatively low risk of complications. It is located on the lateral (thumb) side of the wrist, just below the thumb. It can be palpated easily and is typically the first choice for adults. A modified Allen test is performed to check blood flow prior to a radial artery blood draw. The femoral artery is located in the groin area and is typically accessed by the practitioner in emergency situations or when other sites are not viable. The ulnar artery runs parallel to the radial artery on the medial (pinky finger) side of the wrist. It is less commonly used than the radial artery.
Link to Learning
For the Allen test have the patient clench their fist. Occlude the radial and ulnar arteries with your thumbs Ask the patient to slowly open their hand while you keep pressure on the arteries. The patient’s hand should be pale or blanched from lack of arterial blood flow. Release the pressure on the patient’s ulnar artery. If the patient’s hand becomes flushed, circulation is adequate, and the radial artery may be used. If the patient’s hand remains blanched, circulation is inadequate, and a different site should be used.
Capillary Blood Collection
Capillary blood collection, often used for POC testing, glucose monitoring, and other diagnostic tests, involves collecting a small blood sample from capillaries just beneath the skin’s surface. Capillary blood samples are typically obtained from the fingertips, earlobes, or heel (in infants).
It is often important to keep the patient’s hand warm and in a dependent position to promote vasodilation and obtain a good blood sample. If necessary, warm compresses can be applied for ten minutes prior to the procedure to promote vasodilation. Follow the manufacturer’s instructions to prepare the POC machine (Figure 10.12) for measurement, if using. After applying clean gloves, clean the patient’s skin with an antiseptic wipe for thirty seconds, allow the site to dry, and then puncture the skin using the lancet. If needed, gently squeeze above the site to obtain a large drop of blood. Do not milk or massage the finger because it may introduce excess tissue fluid and hemolyze the specimen. Wipe away the first drop of blood and use the second drop for the blood sample. Follow facility policy and manufacturer instructions regarding placement of the drop of blood for absorption on the reagent strip. Timeliness is essential in gathering an appropriate specimen before clotting occurs or the POC machine times out.
Link to Learning
In this video, how to collect a capillary blood sample to check blood sugar is demonstrated.
Clinical Safety and Procedures (QSEN)
QSEN Competency: Collecting a Capillary Blood Glucose
See the competency checklist for Collecting a Capillary Blood Glucose. You can find the checklists on the Student resources tab of your book page on openstax.org.
Site Selection
Site selection for capillary blood collection is an important consideration to ensure the procedure’s success, patient comfort, and accuracy of the results. The choice of collection site depends on various factors, including the patient’s age, type of test being performed, and ease of accessing the site. Common sites for capillary blood collection include the fingertip, heel, and earlobe (Figure 10.13). The fingertip is the most commonly used site for capillary blood collection in both adults and older children. This site offers good blood flow, is easy to access, and typically causes less discomfort than some other sites. The heel is often the preferred site for capillary blood collection in infants and newborns, especially for newborn screening or routine blood tests. This site provides a larger blood volume for testing and is less painful for the infant. The earlobe is another site used for capillary blood collection in adults and older children, primarily for POC testing. This site may be chosen when fingertip or heel collection is not possible or when the patient prefers it.
Life-Stage Context
Selecting a Capillary Site for Infants
For infants, the primary site for capillary blood sample collection is the heel. When obtaining capillary blood samples in this age group, healthcare providers typically choose the lateral or medial plantar surface of the heel, avoiding the posterior curvature and the center of the heel pad. Due to the sensitivity of infant skin, a gentle and skillful technique is essential to minimize pain and trauma during the procedure.
Documentation of Blood Collection
Proper documentation of blood sample collection is essential for maintaining accurate patient records, ensuring traceability, and providing a clear record of the procedure for healthcare professionals, laboratory technicians, and other involved parties. The blood sample must be labeled with the patient’s information, including patient name, date of birth, date and time of collection, and any other requirements according to facility policy.
Once the blood sample has been drawn, document the procedure within the patient’s chart. Documentation of blood collection should include the following:
- date, time, type, and site of blood collection
- condition of site after blood collection, if appropriate
- laboratory tests for which samples were obtained
- time when samples were sent to the laboratory
- total volume of blood drawn, if appropriate
- any adverse reactions or interventions performed
- the patient’s tolerance of the procedure
- any teaching provided to the patient and family, including understanding of that teaching and any follow-up teaching needed
Patient Education
Patient education is an essential part of the blood collection process and helps reduce patient anxiety, improve cooperation, and ensure that patients understand the procedure, its purpose, and any necessary postcollection care. Be sure to include the following:
- the medical purpose of the blood test
- the steps of the procedure
- how long the procedure will take
- possible complications
- any postprocedure instructions, including signs and symptoms to report to the nurse or provider
For all blood collection routes, clear and open communication is key to ensuring patient understanding and cooperation. Be sure to address any questions or concerns the patient may have.
Interpretation of Results
Interpreting a blood test result involves understanding the reference range, which includes the typical values for a specific test, and recognizing whether the result falls within the range. Blood test results can be categorized into three main groups: normal, high (elevated), and low (decreased). In some cases, a therapeutic range may be referenced, which includes target values for certain tests when managing specific conditions.
Normal lab results are used as a reference to evaluate whether a person’s test values are within an expected range. These ranges are established based on extensive population studies, and they can vary slightly from one laboratory to another as well as with factors such as age, sex, and individual circumstances. Normal results generally suggest that there are no immediate concerns related to that particular parameter.
High or elevated results indicate the value for a specific test is above the upper limit of the reference range. The laboratory results may include an uppercase H or an upward arrow next to the lab value to indicate the result is elevated compared to the normal reference range. High results can indicate various conditions, such as inflammation, infection, organ dysfunction, or other medical issues. It is essential to note that high results may not always be indicative of a severe condition, and further evaluation is needed to determine the underlying cause and potential treatment.
Low or decreased results indicate the value for a specific test is below the lower limit of the reference range. The laboratory results may include a lowercase L or a downward arrow next to the lab value to indicate the result is low compared to the normal reference range. Low results can suggest conditions like anemia, malnutrition, deficiencies, or other health problems. As with high results, low results may require further evaluation to identify the underlying cause and appropriate treatment.
Therapeutic ranges are specific target values used in the management of certain medical conditions. For example, in managing diabetes, the therapeutic range for fasting blood glucose levels might be set at 80 to 130 mg/dL. In this case, the goal is to keep blood glucose within this range to manage the condition effectively. Therapeutic ranges are used to guide treatment decisions and monitor the progress of therapy.
Normal Findings
Blood should be bright red when oxygenated, flow smoothly, and clot within several minutes. Reference ranges for blood tests provide a general guideline as to what the normal findings should be, depending on the test ordered. However, what is considered normal may depend on various factors, including age, sex, and individual health status. Healthcare providers interpret blood test results in the context of the patient’s specific health and medical history to determine the appropriate course of action.
Abnormal Findings
Physical characteristics of blood may help to indicate abnormalities in blood. For example, blood that appears dark red may indicate oxygen deprivation (cyanosis) or hemolysis. Increased viscosity may result from dehydration, while decreased viscosity may be due to anemia or other conditions. Blood that clots too easily may indicate a risk of clotting disorders, while blood that clots too slowly may indicate a risk of excessive bleeding.
Chemical and microscopic testing may also be used to indicate abnormal findings. An abnormal reference range in the context of blood test results typically means that the values obtained for a particular test fall outside the normal range established by the laboratory. Results above or below the normal range may suggest an underlying health condition or problem. For example, elevated liver enzymes (aspartate aminotransferase [AST] and alanine transaminase [ALT]) could indicate liver damage. On the other hand, a low red blood cell count may suggest anemia. Abnormal high or low results often warrant further investigation, including additional tests and a medical evaluation to identify the cause and determine appropriate treatment or management. An abnormal result should always be interpreted in the context of the patient’s overall health, medical history, and any specific symptoms or concerns. Not all abnormal results indicate a serious health issue; some may be temporary and reversible. In some cases, an isolated abnormal result can be due to laboratory error or other transient factors. Repeat testing may be recommended to confirm the abnormality. If an abnormal result is confirmed and is indicative of a medical condition, the healthcare provider will develop a treatment or management plan, which may involve medication, lifestyle changes, or further diagnostic tests.
Clinical Judgment Measurement Model
Prioritize Hypotheses: Interpreting Blood Test Results
Clinical judgment in recognizing an abnormal lab result involves a systematic approach that considers not only the numerical value of the result but also the patient’s clinical history, symptoms, and other relevant information.
A 45-year-old patient, Mr. Suarez, presents to the emergency department with severe abdominal pain and vomiting. He has a history of alcohol abuse. A comprehensive blood panel is ordered, including liver function tests (LFTs). The patient’s LFTs show the following:
- aspartate aminotransferase (AST): 780 U/L (normal range: 10 to 40 U/L)
- alanine aminotransferase (ALT): 890 U/L (normal range: 7 to 56 U/L)
- total bilirubin: 2.5 mg/dL (normal range: 0.2 to 1.2 mg/dL)
- alkaline phosphatase (ALP): 150 U/L (normal range: 44 to 147 U/L)
The nurse notes the elevated AST and ALT are significantly above the normal range, indicating possible liver damage. The total bilirubin and alkaline phosphatase levels are also elevated, suggesting possible obstruction of the bile duct. Assessing the clinical context, the nurse evaluates Mr. Suarez’s clinical history, symptoms, and risk factors. The patient’s severe abdominal pain, vomiting, and history of alcohol abuse are considered. Provided the elevated liver enzyme levels and clinical context, the nurse considers potential causes for the abnormal laboratory findings, such as alcoholic hepatitis, acute pancreatitis, or biliary obstruction.
Factors Affecting Results
Blood test results can be influenced by a variety of factors, including individual, environmental, and technical factors. Common interfering factors are listed in Table 10.10.
| Factor | Effect on Blood Testing |
|---|---|
| Not allowing antiseptic to fully dry | Hemolysis of the specimen |
| Improper tube volume collected (underfilling or overfilling tubes) | Improper additive-to-blood ratio, causing incorrect results |
| Improper CVAD collection procedure | Contamination of specimen |
| Collecting a sample in the same limb as an IV infusion | Contamination or dilution of specimen |
| Using a syringe to collect blood | Hemolysis, overfilling or underfilling of collection tube |
| Excessive probing during venipuncture | Hemolysis, contamination with interstitial fluid, patient nerve injury |
| Improper handling/storage | Incorrect results |
| Incorrect specimen labeling | Incorrect results |
| Leaving the tourniquet on the arm for too long | Hemolysis (lysing of the red blood cells can cause increased potassium levels) |
Patient factors that can affect the results of blood testing include medications, exercise, fasting, malnutrition, dehydration, certain diets, and compliance with the procedure. Time of day can also affect blood results.