A Series of articles to empower you to get optimal lab results by understanding and participating in your lab work.
The most common question I get in the Clinical Laboratory is “When will my lab work be complete?” The second most common, “what does this mean?”. As a decades long lab professional I am prohibited from explaining it to a patient because only the doctor is able to interpret results based on your health and their expertise. So I didn’t answer your question either, did I?
Instead of answering specific questions, let’s look at how a Laboratorian conducts and interprets lab tests. These interpretations are not medical advice, but simple ways to get a “feel” for results and how you can get the best results. There will be a series of articles discussing how “normal” is determined with examples, then we will drop back to some laboratory basics, throw in some regulatory provisions for fun and get inside the beaker of both your body and the lab.
Before we get started, some terms. Analyte: one specific test like glucose. Analytes are usually tested in groups called panels. These panel definitions are guided by federal agencies, examples BMP, CMP and Hepatic Function panels. Prior to CMS streamlining they were Chem 7, chem 8 and chem 12 panels. Panels test analytesin groups to check specific and overall organ function. BMP tests electrolytes and kidney function as well as a glucose . The CMP panel adds tests that check the liver, calcium and greater overall health. Test is used generally to refer to an analyte, several analytes or the determination of those by clinical lab methods by testing. Pathogens, for our consideration, are viruses or bacteria that are causing harm. There are many other pathogens such as fungi, parasites and allergens.
Test results are qualitative, semi- qualitative, semi- quantitative or quantitative. Qualitative tests are positive or negative; present or absent; detected or not detected, like influenza. Semi- Qualitative tests are graded in terms of relative amounts such as negative, small, moderate, or large; they can also be Negative 1+, 2+ and so on, most commonly used in urine testing. Quantitative tests are numerical such as glucose, they have an absolute number and units of measurement, such as mg/dL, mmol/L and so on. Semi- quantitative are drug screen results that are reported as positive or negative but are based on numerical results from the test.
Why do I feel so bad, but my lab work is so good? Simply put, the body is absolutely marvelous at keeping itself “normal”. This is Homeostasis. Britannica helps you visualize these laboratory results as a thermostat.
While the outside influences are different, your body works hard at keeping itself normal to survive by making constant adjustments. Let’s look at some examples. This outside influence also includes pathogens and medications which affect specific organs.
What is my thermostat setting (normal) for body pH? These settings are called reference ranges, reference intervals or normal ranges. These can be age and sex dependent or generalized for all based on the analyte tested. Reference ranges are established by testing normal populations and deriving statistical confidence intervals, or in the case of a viral or bacterial test the absence of pathological agents is normal. 95% of the healthy (normal) population should fall with in the reference interval if there is a range or be negative if there is a pathogen being tested. People on medications which affect the analyze are excluded from normal. See table below for examples of normal.
Analyte | pH | Glucose | Potassium | Troponin | Alkaline Phosphatase |
Feference range | 7.35-7.45 | 70-105 | 3.5-5 meq/L | <10male <15females | 36-150 |
Notes | Arterial blood | mg/dL Fasting plasma | Mmol/L | nG/mL | U/L |
Look at the difference in the numbers between pH this is 0.1 and Alkaline Phosphatase which is 114. This is where homeostasis (normal) becomes important to life. You simply cannot survive 114 points of difference in your pH- laboratorians would call that inconsistent with life. 95 percent of the population’s alkaline phosphatase would never fit into a 36.0- 36.1 range. We are not Homeostasis mechanisms regulate all of these analytes tightly, some tighter than others, often with multilayered mechanisms. The absolute differences in the reference ranges will become very important later as we look to laboratory measurements and error detection / prevention. We will also look at variations of measurements based on the actual method which measures them.
Let’s look at 2 tests for simplicity from the manufacturer to your result. Influenza- a viral pathogen. This is obviously not found in health individuals, therefore the reference interval is Negative or Not Detected. Tests are manufactured to detect a particular influenza. The Manufacturer then tests populations that are both healthy (negative) and determined by another method to have the influenza they want to detect (positive). The other method is a test already approved to detect the influenza and meets FDA standards. Viral tests are generally performed using the antigen- antibody lock and key technique, particularly rapid tests that are done in a hospital or doctors office. The gold standard for detecting viruses is a cell culture, taking weeks to grow. This can be used in manufacturing and testing as a reference method also.
A couple of words about Sensitivity and Specificity. In layman’s terms think of this as a teeter totter.
If a test is too specific it is less sensitive, the reverse is usually true. Keep in mind this is just like the teeter totter at school. It is not directly proportional. If you are in the first grade and a ninth grader is on your teeter totter there is some disproportionality as there is if a preschooler or fifth grader is on it instead. In fact, some tests are intentionally skewed one way or another based on the level of disease we want to detect in an organ like the kidney, or the amount of this analyte present in the body. I want my cancer test to pick up the earliest stage they would deter smaller amounts as a threshold. All lab tests have thresholds at which the analyte is detected. Below the threshold, the test will be negative or not detectable, above the threshold there may also be detection issues, though laboratory techniques and the modern test methods mitigate this. In the end though, the final decision is based on the clinical presentation. This is the case with every individual lab test. In other words- the provider is essential in deciding if the test is accurate based on examination and the provider’s expertise.This is why I can’t tell you what your result means. Sometimes, the provider will add more tests that will look at the same organ or disease, then it’s simple probability. I am sure you have had additional testing.
Sticking with influenza- the desired outcome is every person without the virus tests negative and every person with it tests positive. These are known as true negative and true positive tests and they are related to specificity and sensitivity. They each have an alter ego. Lab folks are not too creative so we simply call them false negative and false positive. That means someone with influenza tests negative and someone without it tests positive. So you may still feel bad with a normal test. How can that be?
The best way to describe the influenza test procedure is lock and key for viral tests. The virus antigen is the key The test used to detect it we will call the lock, it contains an antibody lock that is specific to the virus in your nasal passages. There are many variables in this and it can be far more complex and even interesting as this principle is used not only for viral tests, but other tests such as a Thyroid Stimulating Hormone, Troponin and other tests. Did you know that testing your blood type is a lock and key test?
Before we get too far in the woods let’s throw this train in reverse to the bad guys of the lab, errors. The most common sources of error are found in three phases of testing. So far we have discussed the Pre- analytical phase. This is everything that happens before the test is run. The second phase is the analytical phase this is during the test and before the final result is recorded. The final phase is, you guessed it Post Analytical phase. I did warn you we aren’t too creative. Each phase will be discussed in detail in this series of articles.
Later in the series we will look at a Package Insert from a manufacturer, it’s like the joy of reading nutritional guides. The manufacturer will package inserts to inform the Laboratorian of the proper use for most accurate results. Back to our influenza test- manufacturing is part of the pre- analytical phase. Manufacturing problems are pretty rare, but shipping and storage can occur. See procedural and external controls below. Most common problem areas in pre- analytical phases are: Specimen collection and patient identification (Most errors occur here), storing the specimen correctly and preparing it for the test in a timely manner. Empowerment TIP: Make Sure all of your specimens are labeled with your name and date of birth while you are with the collected- every time. Even if it is just hand written.
Collecting the specimen as the manufacturer specifies: For influenza- the correct swab must be used to ensure the virus is detectable. Just an FYI swabs used for strep throat (a bacterial infection) are different than a viral swab. The influenza swab must be properly labeled uniquely identifying each. patient. The time and date is required because testing must be done within a specific time unless the swab is stored to maintain its viability and keep it free of contaminants. Another error in collection is not getting the swab into the spot where the virus is most likely to be. We all remember the Covid swabs that felt like a brain tickle, they actually hurt. The first spot to detect some respiratory viruses is that area, especially without a runny nose to collect from this area. As tests get more sensitive we can detect smaller amounts and use a nasal swab instead. We may also be able to detect it later as your antibodies are produced but not fully bound to all the antigen sites.
Let’s get into the analytical phase. Test errors occur here most commonly due to lack of training or using expired or improperly stored materials, not correctly timing or interpreting the test or not correctly identifying the patient sample with the test tube or device. You may also feel bad with a negative test because you do not have enough virus to detect because it is too early or too late in the illness. If it is too late in the illness, the antigen will be bound by your body’s antibodies and no binding sites will remain for the test. The good news is, hopefully you are feeling better. A good runny nose will be the best place to find pathogens, in the same manner, that a visibly infected throat will detect strep. For most viral tests- the best detection is during the symptomatic period. We are still talking about positive and negative tests here. If you have a virus such as Hepatitis C, you will know there are other tests that actually quantify the amount of virus inside your body to determine the effectiveness of treatments.
Influenza swabs are not tested directly, they are put into an aqueous solution which will be used to perform the test. This may or may not be a time sensitive step, but the solution must be the correct amount and must be unexpired and properly stored. Side note for how serious we take this phase- Lab timers must be certified annually to ensure they are accurate.We are not using our watch here.) The solution is precisely measured into a cassette or similar device which has the Influenza Antibody plus a color or fluorescent indicator. This is very similar to a pregnancy test, but unlike urine, which is directly measured, the swab’s virus is in solution. There are also Controls built into the test (Procedural Control), like the pregnancy test. This simply ensures the correct amount of solution is applied and for a minimum time. Some tests become falsely positive after a certain amount of time and this will be indicated by a manufacturer statement of “do not read after” with a specified time limit.
Interpreting results can be as simple as a pregnancy test or as difficult as looking at the test tube itself, usually for changes in color. The manufacturer instructions are used to decide if it is positive or negative. There may even be some problems such as a manufacturing error or testing error. The manufacturing errors are almost absent in modern lab. However, if a test was frozen or exposed to excessive heat, it may not perform. For this reason most manufacturers include an external control in addition to the procedural control. A patient who has tested positive or negative with a different test may be used to verify the performance is as expected.
Next let’s look at a urine test for Nitrites. This test is useful for detecting a bacterial urinary tract infection. It’s a quick and simple chemical test that would remind you of using pH paper. This test is part of a panel during urinalysis. These tests are chemical reactions. Time is a critical factor.
The pre- analytical phase is using unexpired test materials, proper collection of urine, testing within 2 hours or refrigerating and labeling with the correct patient. The manufacturer states first morning void is ideal for the panel of testing. This is important because the likelihood ofdetection increases with concentration. I know you have had this test done many times and never first morning void. Who can hold it this long? I have tested specimens at all times of the day with positive results for nitrites, but early infection may theoretically be missed. So you may still feel bad with a negative test. Reference range is negative, in homeostasis terms, your body is designed to reduce urinary tract exposure to these gram negative enteric organisms, found in stool, not in healthy urine. T Remember that false negative result we talked about? False negative may occur if urine is not held in the bladder long enough to reach the detection threshold, if the organism is not nitrate reducing, or if your diet does not contain enough nitrates for the organisms to convert to nitrite, or an interfering color. Note: I find this test to correlate well to the typical gram negative bacteria that cause most urinary tract infections. If this test is positive, I see bacteria under the microscope.
Each test on this urine panel has a limitation of readability affecting the interpretation if there is a substance causing abnormal urine color. If you take a drug which changes your urine color, these tests may be affected. Urine chemistry strips must be kept from light and stored in the original container at the correct temperature until usage. Analytical phase: There are two ways to read this test, visually timed or with a simple instrument that times and reads each pad individually. If read visually, tests are read at different times. Compare the urine dipsticks below- the Far Left dipstick is freshly dipped. The middle dipstick is approximately 1 minute into the test phase. Changes in color from the Left indicate the presence of an analyte. The dipstick on the far right is 5 minutes after the time. You can actually see a change from negative to positive if not read at the correct time.
Fresh Dipped. 1 minute. 5 minutes
Note: some of the analytes are to be interpreted at 30, 60 and 90 seconds. If someone gets distracted and reads the test late, you will get false results. A few manufacturers have mitigated this error by having all the analytes read at the same time, reducing the likelihood of reading too early or too late. After time, the pads on the strip change color and can be misinterpreted.
The correct patient must be associated with each test, proper labeling is required. Post analytical phase: The results must be properly recorded. If this test is manually read, writing down each test at the exact time indicated instead of waiting until latest time to record all results.
I hope this begins to answer your questions on lab results. Your body's beaker is incredible.
References:
Siemens Healthcare Diagnostics. (2017). 10SG Siemens Healthcare Diagnostics Inc. 511 Benedict Avenue Tarrytown, NY 10591-5097 USA www.siemens.com/poc. Tarrytown, New York; Siemens Healthcare Diagnostics, INC.
Encylopedia Britannica (Ed.). (2023, November 17). Homeostasis.
Assessed and Endorsed by the MedReport Medical Review Board
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