Essential laboratory tests for medical education

Medical practice requires physicians to have a broad understanding of the basic sciences, have competent clinical skills, and have an ability to practice in evolving health systems in a cost-effective and evidence-based manner. Essential to the practice of medicine is an understanding of the common laboratory tests and the ability to use them effectively. The Essential Laboratory Tests for Medical Education (ELTME) is a concise document explaining the pathophysiology of common laboratory tests and clinical context for each test and was developed in response to an expressed need from medical students, residents and fellows, and medical educators. The ELTME is linked to the Pathology Competencies in Medical Education and its third competency of diagnostic medicine and therapeutic pathology. The ELTME table is a document of common laboratory tests in alphabetical listing. Laboratory tests may be easily queried by name or organ system, and with simple editing tools, new tables may be constructed to fit the needs of individual curricula or learners. Furthermore, the table may be easily expanded by educators who wish to add specific tests to complement their curricula.


Background
Medical education requires the teaching of knowledge skills and attitudes essential for the competent practice of medicine. The field of medicine has changed rapidly over the last decades, with an exponential increase in the knowledge and skills necessary for effective practice in the modern environment. The skill of understanding laboratory testing is often overlooked, despite the fact that 70% of medical decisions rely on the accurate ordering and interpretation of laboratory tests. 1 While laboratory testing is only a small sliver of the cost of healthcare in the United States, nearly 3/4 of health care costs are influenced by that same laboratory testing. 2 Value-based care is a core component of the Health Systems Sciences 3 curriculum and, therefore, laboratory testing must be viewed in the context of a systems model. Although health care delivery can be seen as one domain and knowledge as another, laboratory testing is integrally involved with both. In health care delivery, laboratory testing plays a role in the clinical domain including diagnosis, treatment, and prevention of diseases, among others. The knowledge domain includes pathophysiology and data collection.
In recognition of the importance of laboratory medicine, the 2017 Pathology Competencies in Medical Education (PCME), 4 dedicated its third competency to diagnostic medicine and therapeutic pathology. The authors recognized that physicians must understand 1) which questions must be asked in the care of the patient; 2) which specific laboratory tests can assist in answering those questions; and 3) how to accurately interpret laboratory test results for optimal patient care and safety. The third PCME competency includes learning goals and objectives across clinical and anatomic pathology including general principles, transfusion medicine, hematology, microbiology, chemistry, immunology, genomics, and anatomic topics including autopsy, surgical pathology, and cytopathology.
Our goal with the Essential Laboratory Testing for Medical Education (ELTME) is to provide educators and learners with an easy-to-use summary of the pathophysiology and clinical context ("clinical pearls") associated with common laboratory tests. We hope this publication will facilitate incorporation of laboratory testing in medical school curricula. We anticipate medical students, residents, and fellows will find this publication useful as a reference to better order and interpret laboratory tests and to provide superior patient care.

Methods
A formal proposal to form a working group that would create a resource for undergraduate medical education was approved by the Association of Pathology Chairs Undergraduate Medical Education Section (UMEDS) Council. One goal of the project was to help medical students understand basic principles of laboratory testing to improve decision making and patient safety. Another goal was to create a useful resource that would allow medical educators to readily incorporate basic laboratory test information into their curricula. The project planned to link common laboratory tests to the third competency of the PCME, and to highlight pathophysiology and clinical pearls of those common laboratory tests.
A question regarding the teaching of laboratory values was posed on the UMEDS listserv July 7, 2020. As the question generated much discussion on this topic, a formal working group was presented to and approved by the UMEDS Council on August 24, 2020. A template for pathophysiology and clinical pearls was created with examples, and the initial laboratory list included approximately 250 laboratory tests generated from a survey of the Mayo Clinic Laboratory testing website. 5 Follow up emails were sent to UMEDS members who had expressed interest in participating in the project in October 2020. Additional tests were added to the list based on the recommendations from UMEDS members. Once working group members had submitted their contributions for each laboratory test, clinical pathology editors and clinical colleagues were identified to review the submissions and recommend changes. Laboratory tests were correlated with the PCME, and revisions condensed the current laboratory test list to 181 common laboratory tests. The table was edited to create a uniform product with similar style and content depth for all entries. Table 1 includes the laboratory tests of the ELTME.
As laboratory tests have varying, population-dependent reference ranges, a uniform reference range that could be used for all included tests was sought. The Mayo Clinic graciously permitted the use of their normal reference ranges for all laboratory tests within ELTME. Common references cited for creation of the table include the Test Catalog from Mayo Clinic Laboratories, 5 Henry's Clinical Diagnosis and Management by Laboratory Methods, 6 Tietz Textbook of Clinical Chemistry and Molecular Diagnostics 7 , Guide to Diagnostic Tests 8 , Robbins and Cotran Pathologic Basis of Disease 9 , and Blood Cells: A Practical Guide. 10 Specific additional references are noted in the table as appropriate.

Discussion
As expected for the scope of this project, the development of the ELTME was a significant undertaking. On one level, this project addresses the essential pathophysiology of common laboratory tests. It was a challenging process to determine which tests would be most relevant to undergraduate medical education. Many laboratory tests, including those in specific areas such as microbiology and genomics, were briefly or not at all covered in an effort to keep the ELTME concise and easy to use.
We anticipate this reference will be a useful resource for medical educators and will facilitate incorporation of laboratory testing into curricula. With the increased adoption of integrated medical curricula, teaching time is often limited, and thoughtful, focused educational materials are essential. Many existing textbooks or references contain more information than a typical medical student will need to know, but due to clinical and administrative duties, medical educators may find it challenging to independently develop curricular materials such as this one. The easy searchability of this table enables educators to quickly identify the tests relevant to the material they are teaching and integrate them into teaching materials as appropriate. With simple editing tools, new tables may be constructed to fit the needs of individual curricula. Furthermore, the table may be easily expanded by educators who wish to add specific tests to complement their curricula.
The second intended audience for this publication is medical students, residents, and fellows who must master an enormous amount of material in a short time. This peer-reviewed, concise document of common laboratory tests addresses what each test measures and the associated pathophysiology. This combination will help provide a more thorough understanding of the basic science underlying disease and laboratory testing. By focusing on the pathophysiology of each laboratory test, the context for educated decision making is provided, thereby contributing to value-based patient care.
The PCME provided a framework for this project on laboratory testing. Medical educators have been using and adapting the PCME for their own curricula since its publication in 2017 and educational cases have been published to highlight the pathology for the PCME learning objectives. By combining the learning objectives for the PCME with corresponding laboratory tests in the ELTME, medical educators and learners can easily query the Association of Pathology Chairs (APC) website (https://www.apcprods.org/journal) to identify associated educational cases for curricular use or self-directed learning.

Conclusion
Physician understanding of common laboratory tests is essential for effective clinical reasoning, diagnosis, and management. The ELTME consists of common laboratory tests essential to basic medical practice and highlights the pathophysiology and clinical pearls for each of those common laboratory tests, facilitating learner comprehension and providing further context. This document is intended to help medical educators, medical students, and resident and fellow physicians understand common laboratory tests, acquire clinical insights, and reference relevant educational cases that illustrate the importance of these tests as well as easily look at the pathology competency and identify educational cases that will further explain or give examples of use of laboratory tests.  The ACh receptor (AChR) is found on the surface of muscle cells at the neuromuscular junction. AChR autoantibodies are found ine85% of patients with generalized and e50% with ocular myasthenia gravis (MG). MG is an autoimmune disorder characterized by skeletal muscle weakness and increased fatigability. Autoantibodies to AChR are classified as binding, blocking, or modulating. Binding antibodies cause complement activation that destroys the AChR. Blocking antibodies prevent the binding of ACh. Modulating antibodies crosslink the receptor subunits, resulting in internalization.

Authors' note
AChR binding antibody titer does not necessarily correlate with disease severity, but changes in titer can be useful in monitoring response to treatment in individual patients. A negative result does not rule out the diagnosis of MG: 30 to 40% of patients with MG who are negative for AChR antibodies express a musclespecific kinase (MuSK) autoantibody. False positives may occur in the setting of thymoma (without MG), Lambert-Eaton myasthenic syndrome, small cell lung carcinoma and penicillamine treatment. 12 Nervous Activated partial thromboplastin time (aPTT), plasma H 2.1 25-37 s aPTT assesses the coagulation factors of the intrinsic (factors XII, XI, IX, and VIII) and the common (factors X, V, II, and fibrinogen) pathway. The test is performed by measuring time to clot formation when a surface activator, phospholipids, and calcium are added to the patient's platelet-poor plasma.
Deficiency of any of the assessed factors can cause elevations of aPTT. When both PT and aPTT are elevated, the deficiency is in the common pathway. A prolonged aPTT should be interpreted in the context of a concurrent PT to determine whether the deficiency is in the intrinsic pathway or in the common pathway. A mixing study may be performed to distinguish a factor deficiency from inhibition. Heparin and antiphospholipid antibodies (lupus anticoagulant) cause isolated aPTT elevation. For most aPTT reagents, the factor VIII activity must be below 35-45% before changes in aPTT are noted. Since aPTT is a clotbased assay, anticoagulation therapy can result in elevated aPTT.  ALT is an enzyme normally present in the cytoplasm of hepatocytes. With plasma membrane injury, it is released and enters the blood. Low levels of ALT may be released with damage to the kidney and skeletal and cardiac muscle. ALT is more specific for the liver than aspartate aminotransferase (AST).
Both ALT and AST levels increase in liver disease; however, ALT is more specific for liver injury and remains elevated longer than AST. Increases in ALT may precede symptom onset. In inflammatory conditions of the liver (e.g., acute viral hepatitis, autoimmune hepatitis), ALT levels are usually equal to or higher than the increase seen in AST, resulting in an ALT:AST ratio of more than 1. In the setting of excess alcohol use, AST is elevated to a greater extent than ALT, leading to an AST:ALT ratio of >2. In endstage liver disease, both enzymes may be low due to massive tissue destruction.     In recent years, there has been increasing literature regarding the significance of serum anti-52-kD Ro and anti-60-kD Ro; the former has a reported association with congenital heart block and the latter is more often present in autoimmune disease.  . In an antibody screen, a patient serum sample is mixed with reagent RBCs with known antigen profiles. When the antibody screen is positive, the blood bank identifies these antibodies. This process may also be performed for platelets. AST is an enzyme normally present in the cytoplasm and mitochondria of hepatocytes. With hepatocyte membrane injury, it is released and enters the blood. In addition to the liver, AST can also be released from the kidney, skeletal muscle, and the heart. Alanine aminotransferase (ALT) is more specific for the liver than AST.
AST is increased in liver disease, toxic injury, or viral infections of the liver. In the setting of excess alcohol use, AST is elevated to a greater extent than ALT, leading to an AST:ALT ratio of >2. Both ALT and AST require vitamin B6 for activity, but ALT is more dependent on vitamin B6, which is often deficient in individuals who chronically drink excessively.   In response to ventricular wall stretch and volume overload, cardiac myocytes cleave an Nterminal section from the BNP prohormone (NT-proBNP) to release active BNP and the inactive N-terminal fragment, NT-proBNP. BNP downregulates the renin-angiotensin-aldosterone system, decreases sympathetic tone in the heart and kidney, and increases renal blood flow and sodium excretion. NT-proBNP has a longer half-life than BNP.
NT-proBNP and BNP levels are useful in distinguishing acute onset dyspnea secondary to congestive heart failure (CHF) versus lung disease since it is elevated in the former, but not the latter. However, NT-proBNP should not be used in isolation to establish the diagnosis of CHF.
Cardiac C-reactive protein (CRP), serum IMM 1.1 8 mg/L CRP is an acute phase reactant that acts as an opsonin. In acute inflammation, IL-6 stimulates production of CRP from hepatocytes. CRP is a sensitive but non-specific marker of inflammation. It has a short halflife (hours).
CRP is increased in a variety of acute illnesses and inflammatory conditions (e.g., bacterial infection, myocardial infarction). Higher baseline levels of plasma CRP are associated with increased risk of chronic heart disease and stroke, possibly through the inflammatory response associated with atherosclerosis. Assays to measure the low baseline levels of CRP are often referred to as 'high-sensitivity' or 'cardiac' CRP assays to distinguish from regular CRP assays that measure the much higher levels seen inflammation or infection.         Ferritin is found in serum and in the cytoplasm of tissue macrophages; it is the major storage protein for iron. Ferritin concentration varies with age and sex and correlates with total iron stores; therefore, ferritin levels are low in iron-deficiency anemia and high in iron overload (e.g., hemochromatosis). Ferritin is an acute phase reactant and can be increased in acute and chronic inflammation as well as in chronic kidney disease and some malignancies.
Ferritin is often measured in combination with serum iron, transferrin saturation and total iron binding capacity; these tests may be less precise and do not distinguish depleted iron stores from impaired iron release (e.g., anemia of chronic inflammation). Low serum ferritin is highly specific for iron deficiency anemia. However, acute phase reaction can sometimes mask the low ferritin that would otherwise be seen in iron deficiency anemia. 37         The prothrombin time (PT), which is used as a screening test for coagulopathies, shows intra-and interlaboratory variation due to differences in instruments used and variable tissue factor activity.
To correct for these differences, the INR is calculated by dividing the patient's PT by a control PT with a standardized thromboplastin reagent.
INR is most commonly used as a means to monitor response in patients who are being anticoagulated with vitamin K antagonists (e.g., warfarin). It is also used to assess patients with bleeding diatheses secondary to defects in the extrinsic pathway, in disseminated intravascular coagulation and to monitor patients with end-stage liver disease.  Most lead is absorbed via the gastrointestinal tract and can be distributed throughout the body, with some preference to bone, erythrocytes and keratin-rich tissues. Adults typically absorb 1-10% of the ingested amount; this increases to up to 50% in children, particularly if they have coexistent nutritional deficiencies. Lead forms covalent bonds with protein cysteine sulfhydryl groups; this contributes to renal toxicity and accumulation in keratin-rich hair. Lead decreases heme biosynthesis and acts as a mitochondrial toxin.
Clinical manifestations vary by age. Children may show decreased nerve conduction velocity, decreased vitamin D metabolism, abdominal pain, anemia, nephropathy and encephalopathy. Lead incorporated into the physes can be seen radiographically as "lead lines". In adults, initial presentation is typically elevations of systolic pressure and decreased hearing, progressing to peripheral neuropathies and nephropathy, and finally to anemia and encephalopathy at later stages. 26 Toxicology Lipase, serum CHEM 1.2 13-60 U/L Lipase is a digestive enzyme produced in pancreatic acinar cells and secreted into the duodenum to digest lipids. With acinar cell injury (e.g., acute pancreatitis), lipase can be released into the pancreas itself where it contributes to local tissue damage, including acute inflammation, autodigestion of pancreatic parenchyma, fat necrosis and vascular damage.
Serum lipase is elevated in acute pancreatitis and is a more sensitive and specific test than serum amylase. In this setting, serum lipase is typically elevated > three times the upper limit of normal.   Monocyte numbers increase in chronic infection (e.g., tuberculosis), chronic inflammation (e.g., autoimmune disease) and myeloid neoplasms (e.g., chronic myeloid leukemia). Monocyte levels may be decreased in inherited immune disorders, corticosteroid therapy, chemotherapy, some infections and hairy cell leukemia.
CBC N-type and P/Q-type calcium channel antibodies, serum CHEM 1.6 N-type: 0.03 nmol/L P/Q type: 0.02 nmol/L N-type or P/Q type voltage-gated calcium channels (VGCCs) are expressed on presynaptic axon terminals and regulate presynaptic neurotransmitter release. Antibodies against these proteins mediate cross-linking of channel subunits, reducing channel cell surface expression and thereby attenuating channel activity. Of the two anti-VGCC antibody subtypes, anti-P/Q antibodies are more common.
Anti-VGCC antibodies are seen in Lambert-Eaton myasthenic syndrome (LEMS), an autoimmune disease of presynaptic terminals; the majority of cases (50-60%) occur in association with small cell lung cancer. Impaired neurotransmitter release from the lower motor neurons, sympathetic neurons, and parasympathetic neurons in LEMS results in weakness, reduced tendon reflexes and autonomic dysfunction.       The RBC count is increased in polycythemia and decreased in anemia. In thalassemia trait, the RBC count is often higher than expected for the degree of anemia. Polycythemia is due to increased production (e.g., polycythemia vera, EPO administration, EPO-producing tumors), whereas anemia may be due to decreased production (e.g., iron deficiency, infiltrative marrow processes) or increased destruction (e.g., sickle cell anemia, hereditary spherocytosis). Inaccurate RBC counts may be due to 40 CBC (continued on next page)    In the serum, iron is bound to transferrin, which is typically about one third saturated with iron. When iron stores in the body are depleted (e.g., iron deficiency anemia), transferrin levels increase in the blood, which increases the total iron binding capacity (TIBC).
In iron deficiency, TIBC is increased, since transferrin levels are relatively high compared to iron content; in iron overload, TIBC decreases since the free transferrin diminishes. TIBC, serum iron and percent saturation are often assessed in the setting of iron deficiency anemia; however, serum ferritin is more sensitive and more accurately reflects the body's iron stores.  Troponin is a regulatory protein of striated muscle composed of three subunits: T, C, and I. The T, or tropomyosin-binding subunit, binds to muscle fibers. cTnT is specific for cardiac muscle and is released in myocardial cell death, about 2-3 hours after acute myocardial infarction. Because cTnT is bound to muscle fibers, it is released slowly into the peripheral blood following myocardial infarction. Its concentration peaks at 24 hours but it can persist 2 weeks or longer in the blood.
Hs-cTnT assays are preferred over conventional tests, which are relatively insensitive; for example, the former are reported as ng/L while the latter are reported as ng/mL. With hs-cTnT tests, low levels of troponin are detectable in healthy individuals. In addition to myocardial infarction, elevated cTnT can be seen in cardiac contusion, congestive heart failure, renal failure, pulmonary embolism and myocarditis. Biotin (vitamin B 7 ), which is found in many multivitamins, interferes with the assay and can cause inaccurate results. 55,56 Cardiac Uric acid, serum CHEM 1.5 Males: < 8.0 mg/dL Females: < 6.1 mg/dL Uric acid is generated by purine metabolism. Purines are synthesized by the body or are ingested, particularly in foods with abundant nucleic material (e.g., liver). About 75% of the body's uric acid is excreted in the urine.
Hyperuricemia can be seen in patients on cytotoxic drug regimens (e.g., cancer chemotherapy) and in the context of gout, leukemia, chronic renal failure (decreased excretion) and psoriasis.