Black communities are already faced with substandard healthcare, so when a medical device that monitors vital signs, such as the pulse ox, is developed without Black and dark-skinned people in mind, it is much more than just a design flaw.
In our last blog post, we went into detail about pulse oximeters and their mechanism of action. As we touched on briefly, blood oxygen measurement varies greatly from person to person since it relies on light passing through tissue, which behaves differently depending on a number of biological factors. This seemingly objective method actually serves as an example of biomedical technology that fails to account for all of these factors.
Low oxygen saturation levels, the type of sensor in the device, gender, and skin color have all been shown to cause errors in pulse oximetry, but the discrepancies relating to skin color may be the most glaring. Studies and extensive anecdotal evidence from clinicians have shown that any errors due to low oxygen saturation were more dramatically skewed in Black patients when compared to their white counterparts. More specifically, at oxygen saturation levels below 80%, pulse oximetry measurements are significantly overestimated in dark-skinned patients.
At saturation levels below 80%, patients begin to experience oxygen deprivation to the point of organ failure (hypoxia). So, if a pulse ox reading is overestimated in a dark-skinned patient, a healthcare professional could easily miss the onset of hypoxia. This limitation of the technology is critical especially now, during the COVID-19 pandemic, which is killing Black people at a rate three times higher than that of any other racial group in America. Black communities are already faced with substandard healthcare, so when a medical device that monitors vital signs, such as the pulse ox, is developed without Black and dark-skinned people in mind, it is much more than just a design flaw.
As engineers, we are taught that device design of any kind cannot be successful unless the product meets all necessary requirements. Just like designing a building requires every environmental condition to be taken into consideration, designing a medical device requires every kind of person to be taken into consideration.
Nevertheless, in the 1980s, when the pulse ox first underwent FDA screening, accuracy and calibration testing was conducted primarily on light-skinned people; this has not changed since then. Despite the major functional disparity, the FDA has yet to require further research and testing dedicated to developing a pulse ox that produces accurate measurements on darker skin.
With each passing week, we are learning more and more about how to deal with this pandemic, both individually and as a community. We are now well-versed with preventative measures like washing our hands frequently and wearing masks, but what happens if you actually start feeling sick? While the symptoms of this viral infection are varied, they usually include high body temperatures, dry cough, and shortness of breath. Most of us have a thermometer at home, with which we can easily diagnose abnormal temperatures. But, is that enough to detect the early stages of a COVID-19 infection?
Pulse oximeters (pulse ox) are getting a lot of attention right now. If you have ever had surgery or if you have a respiratory condition like asthma, you likely know that a pulse oximeter is the little medical device that clips onto your finger and informs your doctor of your heart rate and how much oxygen is in your blood. Monitoring blood oxygen levels has been critical for COVID-19 patients because a drop in the amount of oxygen in your blood indicates the need for more aggressive interventions. Since you can buy a pulse oximeter at the drugstore, many people are wondering if they need one at home. So why exactly do respiratory issues warrant the use of a pulse oximeter?
When your lungs are functioning properly, around 95% – 98% of the blood in your arteries should be “oxygenated,” or carrying oxygen. Your blood carries oxygen with the help of hemoglobin, a protein that has the ability to bind to oxygen molecules. Hemoglobin is what makes blood a great transporter of oxygen from your lungs to the other organs in your body. Without oxygen, your organs cannot function because they rely on a process called oxidative phosphorylation, which uses oxygen to produce the energy that drives all organ functions. When your lungs are compromised, like they are during a COVID-19 infection, they are unable to efficiently take oxygen in from the air and pass it into your bloodstream. As a result, your other organs don’t get enough of it to do their jobs. This condition is called hypoxia.
Now, let’s get back to the pulse oximeters. A pulse oximeter measures the percentage of oxygen saturation in your blood by shining both a red light and an infrared light into the top of your finger. The bottom of the device has a sensor that detects the amount of light that passes all the way through your finger. You can visualize this mechanism by doing a quick little science experiment on yourself. Turn on your cell phone flashlight, put your finger on it, and see what happens. If red light shines through, your blood is probably not deprived of oxygen. Good for you! Oxygenated blood absorbs every wavelength of visible light except red, which is why the red light can go all the way through your finger. Deoxygenated blood, however, is really good at absorbing red light. Now, we can see how the pulse oximeter takes advantage of these properties of blood to give a measurement of oxygen levels.
So, why is infrared light also necessary? Blood vessel width varies from person to person, making the volume of blood in the vessels vary as well. Only using red light would result in misleading oxygen levels because the readings would be affected by these varying blood volumes in different people. For that reason, infrared light is used alongside red light to normalize the measurement and adjust it to each user’s body. Infrared is not well-absorbed by either oxygenated or deoxygenated blood, so it is a good baseline comparison measurement. The pulse oximeter calculates the ratio of absorbed infrared light to absorbed red light to get the percentage of blood saturation. In oxygen-rich blood, the low level of infrared absorption divided by the similarly low level of red absorption results in a ratio close to 1, which is equivalent to a percentage close to 100%. As blood oxygen levels decrease, this ratio also decreases because the red light absorption (the denominator) increases while infrared absorption stays relatively constant. Readings below 92% indicate the beginning of a hypoxic state.
The ability to detect hypoxia is what makes pulse oximeters critical for COVID-19 patients. Healthcare professionals who are currently treating these patients are finding that oxygen levels can drop well below 92% before patients have any trouble breathing. When a patient does finally go to the doctor complaining of shortness of breath, their infection and hypoxia may be very advanced. Often, symptoms like fever and fatigue can also mask early hypoxia symptoms. It is easy to get used to the mild shortness of breath or pass it off as fatigue while blood oxygen levels continue to drop. For these reasons, some doctors are suggesting that everyone should get a pulse oximeter to use at home so the onset of hypoxia can be caught early.
A big disclaimer that many physicians are making however, is that pulse oximeters should be used at home along with thermometers and calls to your doctor. There are still many cases in which COVID patients do not present with any hypoxia, but have high fever and the telltale dry cough. So, blood oxygen level, while helpful, is not the only metric used in diagnosis.
So, should you buy a pulse oximeter? To put it plainly, it’s really up to you. If you are sick, you called the doctor, and they said you’re not sick enough to go to the hospital, it may be helpful to have one and monitor your own blood oxygen so you know if you ever do need to go in. You may want one in that situation just to keep your peace of mind. Even if you aren’t sick, and just want to be prepared, it definitely can’t hurt to get one. However, all the stories about hypoxia going unnoticed paint a scary picture. It’s important to remember that, if you can’t get your hands on a pulse oximeter right now since the demand is high, it’s not the end of the world. The vast majority of COVID patients are able to get diagnosed and get to the hospital in time if necessary just by calling a doctor. So, you really don’t need to buy that $200 pulse oximeter you found on Amazon. You can find them for about $30 at your pharmacy, but if they’re sold out, don’t panic! Just keep washing your hands, keep that 6-foot distance, and if you are sick, call your doctor. Here at the blog, however, we are device geeks – so any opportunity to have a new medical device around the house we’ll take!
After spending the last few weeks talking with students, journalists, neighbors and family members, I’ve decided that there might be some value in discussing some of the terminology that we use when talking about diagnostic testing.
Science communication and media relations folks always discourage “jargon” when describing scientific concepts to laypeople. However, technical terminology can be a key part of scientific discussions, especially when it’s important to be precise. Now, in the setting of a global pandemic, precision is particularly important, and I believe people can handle more complexity than the scientific community often gives them credit for. Increasing scientific literacy can empower people to better understand and digest current events. So, here are some of those definitions.
First off, is reagent. In general, a reagent is any substance that is a starting material for a chemical reaction. Do you remember finding the “limiting reagent” in high school chemistry class? That’s the chemical that runs out first; thus, limiting the amount of product that can be made by the reaction. Many outlets have reported that one of the reagents failed in the initial test that the CDC distributed for COVID-19. It is still unclear which reagent did not perform as expected, but the reagent was one of the parts of the test reaction that the CDC shipped out to labs around the country. This problem has now been fixed, and all of the CDC test components are working well.
The next term is assay. Assay is just the word we use for “test.” Anytime you hear someone say they are running an assay or assaying for something, they are simply running a test.
Controls and control material also come up often when discussing test design. Controls are needed to make sure that the test you are running is valid. Basically, controls are parallel experiments that you run alongside your testing to make sure that you didn’t make a mistake while running the test. We science folks are always skeptical and are always checking to make sure we didn’t make a mistake! A positive control, in the context of COVID-19 testing, is when we put some material in the reaction that we know will make the test come up positive. If that reaction does not come up positive, we know that we made a mistake someplace, or that some of our other reagents are not working properly. A negative control is when we set up a test that does not contain a sample or additional material, that we expect to come up negative. If a negative control comes up positive, then we know that we have some kind of contamination, or that something we did not expect is happening in our reaction. If this happens, we need to start over, and figure out what went wrong. In both cases, controls that do not work as expected render any test result invalid. These results are not reliable and should not be reported.
Now, we need to cover sensitivity and specificity. These guys are a bit more complex. Sensitivity measures how little of the virus you can detect using a particular test in a particular sample. A super-sensitive test can detect very small amounts of the virus. Sensitivity goes hand in hand with the concept of a false negative. A test that is not sensitive enough might come out negative for someone who is infected with the virus, but does not have a viral load high enough to make the test read positive. It is also possible that a false negative can arise from a swab being taken improperly.
Specificity is a measure of how well the test detects the COVID-19 virus, and not other things that might confuse the test. A very specific test is good at detecting COVID-19 and will not detect other closely related viruses. Specificity goes hand in hand with false positive rates. If a test is not very specific, it might show a positive result when someone is not infected with COVID-19, but with something else like a flu virus.
Both false positives and false negatives make it difficult for healthcare providers to interpret test results in the context of care. Low rates of false positives and false negatives make a test more trustworthy.
Now you know the basics of test design and metrics! We hope that this makes reading some of the science reporting a little more clear.