POL testing is a critical component of patient diagnosis and management
By Jim Poggi
Inflammation, infection and tissue damage is a common cascade of events in the presence of many diseases. Sometimes the initial inflammation can be subtle and provide no overt symptoms until the underlying condition is well advanced. Gingivitis and certain heart conditions come to mind. Internal infections including H. pylori also fall into this category. Autoimmune diseases and arthritis typically present as inflammation initially.
In this month’s article, I will review some of the common tests available today to help diagnose these conditions and to provide laboratory information to shorten the course of the disease and improve patient outcomes. For diagnosis of these conditions, with the exception of acute myocardial infarction (heart attack), testing at the point of care is the first line of defense and POL testing is a critical component of patient diagnosis and management.
When the body comes under attack from viruses, bacteria, or other foreign agents, there is a pretty typical cascade of events. First, there is an inflammation of the affected tissue. In the early going, it may remain subclinical with no apparent symptoms. The affected tissue does not usually become distended, warm to the touch or have other obvious symptoms. This is especially true of internal infections, including arthritis and autoimmune diseases where the source of the inflammation is less apparent. Over time, inflammation commonly turns to infection, further impacting the tissue under attack. Finally, under extreme conditions, the muscle dies. Endocarditis and ischemic heart disease often result in damage to the heart muscle.
Inflammation markers
C-reactive protein (CRP) is a common screening test performed to rule out inflammation and is also often used as a first screen for arthritis, either alone or in combination with rheumatoid factor (RF). While it is highly sensitive, it is not a specific indicator of a specific disease. As a result, a screening positive result on CRP will lead to further tests including erythrocyte sedimentation rate (ESR), CBC, autoimmune tests including RF, antinuclear antibody (ANA) and others. There are multiple CRP methods available, ranging from simple latex agglutination slide tests to meter based tests all the way up to chemistry instruments. The range of expected values for quantitative tests is from 0.8 to 10 mg/L with most experts considering a level above 3 mg/L as abnormal.
High level CRP tests have become widely available on a number of chemistry systems, and are more commonly used in diagnosis of heart disease. Typical reference range is between 0 and 3 mg/L with levels above 1.0 mg/L considered abnormal. Both CRP and hs-CRP tests are CLIA moderate complexity.
Infection markers
An elevated CRP test is typically the gateway to further lab testing to pinpoint the underlying condition and initiate an appropriate treatment protocol. ESR testing often accompanies CRP and is also a non-specific indicator of infection. Reference ranges vary between genders and increase with age. Women tend to have higher ESR values than men irrespective of age. ESR is also moderate complexity and both manual and automated test methods are available.
When differential diagnosis of infection is warranted, a complete blood count with specific attention paid to WBC count and WBC differential results is a classic follow up to CRP or ESR. We’re all familiar with the range of hematology systems available, ranging from even waived systems to automated hematology systems with 5-part differentials and specialized hematology parameters, including monocyte distribution width, which has recently been shown to have diagnostic value for sepsis and reticulocytes. Due to their versatility, they play a key role in diagnosis of diseases ranging from infection, anemia, bleeding disorders and cancer. Getting to know your hematology suppliers and their products well is an excellent strategy to grow your lab business and build long standing customer relationships.
Sepsis is sometimes first encountered in primary care, but is rapidly escalating to tertiary care centers, usually through the ER. In the specialized world of diagnosis of sepsis, procalcitonin, lactate and monocyte distribution width are common follow up tests used to aid in the diagnosis and lead to treatment plan initiation.
Muscle damage markers
Untreated infections as well as acute myocardial infarction (heart attack) can result in sudden, acute muscle damage. Rapid diagnosis and initiation of an appropriate treatment plan are critical steps to reduce the amount of muscle lost and, in the case of heart muscle, retain reasonably normal heart function. While muscle damage markers are typically performed in the tertiary care hospital setting, the urgent care and free-standing ER customers we call on also see patients presenting with symptoms that may require muscle damage tests. The time-honored test to diagnose heart attack is CK-MB. In health, CK-MB levels are undetectable. For diagnosis of heart attack, it is typically used in combination with its parent enzyme, CK. When an elevated CK-MB along with a CK-MB:CK ratio of 2.5-3.0 is encountered, heart attack is a probable diagnosis.
Troponin I and T (two different assays) are newer methods to diagnose heart attack and acute coronary syndrome (insufficient blood flow to the heart). Range of expected values for Troponin I is 0.0 to 0.04 ng/ml. Values above this level indicate a possible heart attack. Recently high sensitivity Troponin tests have entered the market. They are believed to be even more useful for acute coronary syndrome in particular due to their higher sensitivity. The range of expected values for hs-TnI is 0.0 to 0.14 ng/l. Troponin is CLIA moderate complexity and typically performed on immunoassay instruments.
The last muscle damage marker for this discussion is lactate dehydrogenase (LD). It is elevated when damage occurs to a number of organs including the heart, liver and kidneys, and can also be elevated in sepsis or certain anemias. The range of expected values for LD decreases with age and is less than 40 U/L in adults. It is CLIA moderate complexity and typically performed on chemistry systems. Since it is not as specific as other organ damage markers, it is often performed with other tests including liver enzymes (ALT, AST) and comprehensive metabolic panel to try to identify the organ with damage.
Knowing some of this important clinical information helps keep you in the know as a solid consultative resource for your customers. Use it wisely and good luck.