A lab manager recently emailed me and asked my opinion about whether it was okay to use generic mouthpiece filters on their test systems. They had asked the same question of their equipment manufacturer and received the following statement (parts of which have been redacted by me):

“The [model number] PFT system was designed/tested/certified using the [manufacturer’s] filter. While other “off-label” filters may fit our devices, they have never been tested or approved for use by [the manufacturer]. The precision and accuracy of our devices could be compromised by using different type filters. It is our recommendation that you continue to use the [manufacturer’s] approved filters with your PFT equipment.”

Since I doubt the manufacturer has tested their equipment with any other mouthpiece filters than those they sell this is in some ways a true statement. Having said that, it is also a statement designed to sow fear, uncertainty and doubt (FUD) in the minds of their customers about a subject that is relatively straightforward.

The human respiratory tract is a potential source of particles in the 0.1 to 20 micron range, particularly when coughing but even to some extent during quiet breathing. Mouthpiece filters are barrier filters and intended to prevent these particles from getting into PFT equipment. Filter manufacturer’s claims are very similar and usually state a “Bacterial filtration efficiency: > 99.999% and Viral filtration efficiency: > 99.99%”. In one sense this statement is somewhat disingenuous because mouthpiece filters are not tested with bacteria or viruses (which have diameters as small as 0.03 microns) directly, but are instead tested with aerosols generated by a nebulizer.

A HEPA (High Efficiency Particle Absorption) filter is a true bacterial filter and to meet standards it must filter out 99.97% of all particles 0.3 microns or larger. Mouthpiece filters are not HEPA filters, partly because of cost but far more importantly because HEPA filters have a lot of resistance to air flow. A HEPA filter is a sieve mouthpiece with opening sizes that prevent particles above a specific size from passing through. Mouthpiece filters instead work by impaction and electrostatic attraction. Larger particles are captured by impacting or otherwise being intercepted by the filter fibers and the fibers usually also have an electrostatic charge that attracts smaller particles.

Nelson Labs (Utah), which does much of the mouthpiece filter testing in the USA, describes their Increased Bacterial Filtration Efficiency (BFE) test features as follows:

“The test is conducted using Staphylococcus aureus as the challenge organism. A liquid suspension of S. aureus is aerosolized using a nebulizer and delivered to the filtration medium at a constant flow rate of 30 liters per minute (LPM).

“The aerosol droplets are collected in all-glass impingers (AGIs) in parallel. The challenge is delivered for a 1-minute interval and sampling through the AGIs is conducted for 2 minutes to clear the aerosol chamber. The titer of the assay fluid is determined using standard plate count and/or membrane filtration techniques. The number of bacterial aerosol droplets contacting the filter medium is determined by conducting challenge controls without filter medium in the test system. Challenge controls are maintained a 1 x 10^6 colony forming units (CFU) with a mean particle size (MPS) of 3.0 +/- 0.3 microns. This allows filtration efficiencies to be reported up to >99.9999%”

Since any bacteria or viruses exhaled by the human respiratory tract are contained within liquid droplets this test does have a basis in fact, but what is being tested is the ability of mouthpiece filters to remove particles that are much larger than individual bacteria or viruses.

Mouthpiece filters come in a variety of shapes, sizes and there are probably differences in the actual filter material but the two factors that are most important are resistance and deadspace volume. The ATS/ERS standards for spirometry states that the total resistance of flow in a spirometer should not be greater than 1.5 cm H2O/L/sec. Since the flow resistance of PFT equipment is more-or-less fixed, in order to keep the overall system resistance low, the resistance of mouthpiece filters must be kept low as well.
In reviewing different manufacturer’s mouthpieces I’ve found a range of resistances from 0.08 cm H2O/L/sec to 0.75 cm H2O/L/sec, with most clustering around 0.45 cm H2O/L/sec. Resistance is not the same at all flow rates however, and in fact increases with increasing flow rates. For this reason resistance should be stated at the maximum flow specified in the ATS/ERS spirometry standards (14 L/sec). I’ve noticed that the lowest filter resistances also tended to be specified at flow rates well below 14 L/sec (in one case at 0.5 L/sec) and this is probably misleading.

I’ve found filter deadspaces ranging from 35 to 75 ml. I would have thought there would be a relationship between deadspace volume and filter resistance because the way to keep resistance low is by increasing surface area which in turn requires a larger volume. What I found instead was that that some filters with low deadspace had a low resistance and some filters with a larger deadspace also had a higher resistance. The differences in resistance and deadspace may be due to the choice of filter medium rather than surface area, but given the way in which resistance was often reported this isn’t as clear as it might be.

Filter mouthpieces can affect test results. A couple of studies showed slight but statistically significant decreases in FVC, FEV1 and SGaw when a mouthpiece filter was used compared to when it wasn’t. The differences were quite small however, and the studies concluded that any decrease in measured values was not clinically significant.

The most important question however, is whether mouthpiece filters actual prevent equipment contamination and the cross transmission of respiratory infections. More than one study has showed that without mouthpiece filters spirometers are capable of rapidly becoming contaminated. It is much less clear whether or not contaminated equipment is capable of cross-infecting patients but the potential is clearly there. Even if patients aren’t at risk there have been reports in the past of technicians who likely acquired infections (including tuberculosis) from contaminated equipment.

The evidence concerning the ability of mouthpiece filters to prevent equipment contamination is a bit more equivocal. Several studies have indicated there is no difference in contamination rates with or without filter mouthpieces but there are also several studies that showed no contamination when a filter was used compared to when it wasn’t. Strictly speaking there is no overwhelmingly clear evidence that mouthpiece filters prevent equipment contamination but comparison of these studies is difficult because of the different methodologies that were used.

This may also be in part because the Bacterial Filtration Efficiency test used to assess mouthpiece filters has some significant limitations. The test is performed at a flow rate of 30 LPM which is only 0.5 L/sec. The peak flows obtained during spirometry are usually much higher than this and it is not clear that efficiency ratings obtained at a low flow rate extrapolates in any way to higher flow rates.

Nevertheless, all the standards for clinical, office and occupational spirometry that I’ve been able to find recommend the use of mouthpiece filters. My personal opinion is that even though mouthpiece filters may not completely prevent equipment contamination, they certainly reduce it. At least one study has showed it was possible to prevent contamination and cross-infection without filter mouthpieces but this was only possible through regular and diligent cleaning and disinfection that had a cost in technician time and machine down-time well above what would have been spent on filters.

Finally, one additional reason to use filter mouthpieces other than preventing contamination is that when they are upstream of flow sensors they act as diffusers and should act to improve flow laminarization. This may improve the accuracy of flow sensors but this is a personal observation and to my knowledge is not something that has been studied. Even so, it is probably a best practice to calibrate spirometer and flow sensors using the same filter that is being used for patient testing.

Manufacturer “approved” filters are usually more expensive than “off-label” and most equipment manufacturers have good reasons to want to sell their “approved” filters. Their manuals and websites often contains language that implies that their equipment works better with their “approved” filters but since I’ve seen no information from any manufacturer that explains why “off label” filters should not be used with their equipment this is mostly FUD. Interestingly, details concerning the resistance or deadspace of “approved” filters is often hard to find (when it is available at all). In addition, I’ve noticed some misleading information such as a claim that filters were 100% effective (not possible for even HEPA filters) or that resistance was 0.10 cm H2O at 0.5 L/sec (what was it at 14 L/sec?). One final point is that I doubt whether any PFT equipment manufacturer actually manufactures their “approved” mouthpiece filters and it is far more likely that they are manufactured by the same companies that make the “off label” filters.

So, rather than FUD, mouthpiece filters should instead be evaluated on:

• Does it fit the equipment?
• Has their efficiency been tested?
• Has their resistance been measured at (or at least near) 14 L/sec?
• Deadspace.
• Can the filter be used for routine spirometry without any additional adaptors?
• Can the filter accommodate more than one size of flanged rubber mouthpiece?
• Is the price (and shipping costs) right?

Mouthpiece filters may differ from one another in resistance, deadspace volume and filter material but most of them have similar BFE filtration capabilities. Any differences that occur when “off label” filters are used are far more likely to affect patient test results due to differences in resistance and deadspace than they are to affect any manufacturer’s test equipment. I have used filter mouthpieces for as long as they have been available. Admittedly there have been periods where I have used manufacturer-approved filters but those times were either when they were priced competitively or for one short interval, where I couldn’t find any “off label” filters that fit our equipment. Most of the time however, I have used “off label” filters and see no reason for any other PFT lab not to do the same.

 

References:

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