## 3 Clinical Significance of CPF Evaluation  
### Conventional CPF Measurement Methods  
Figure 2 shows examples of conventional CPF measurement devices. Various spirometers [3, 23, 24[PERSON] 25, 26] and peak flow meters [24, 26, 27, 28, 29, 30] are used for CPF measurement. Moreover, various flow sensors, such as a pneumotachograph [21, 23, 24, 25, 31] that measures respiratory flow using differential pressure sensors, a constant temperature hot-wire flow sensor (Figure 2a) [26, 32] and an ultrasonic sensor [33], are used in spirometers. In particular, the peak flow meter that can measure the flow rate using a spring-loaded piston is most commonly used because of its small size and ease of use, enabling patients to measure CPF by themselves (Figure 2b). However, [PERSON] [34] raised a problem with the accuracy of various peak flow meters compared with the that of the pneumotachograph and called attention to the following points. It is important to recognize that, depending on the measurement instrument, absolute values of CPF reported in literature may not be directly comparable. Similarly, peak flow meters should be used with caution when measuring CPF in clinical practice, particularly with respect to directing patient management, as this can be considered an off-label use of these instruments [34]. On the other hand, the reliability and validity of some peak flow meters were verified. Thus, it is necessary to understand the characteristics of various CPF measurement devices and select the appropriate device depending on the user's purpose.  
There are two types of accessories for CPF measurement: a mouthpiece and a face mask. These accessories are attached to the peak flow meter or the flow sensor of the spirometer. Figure 2c shows the face mask. Clinically, the face mask is used instead of the mouthpiece for CPF measurement because patients with neuromuscular diseases have difficulty holding the mouthpiece with their mouth while they cough, and cough air leaks from the corner of the mouth. The use of different mask sizes may influence the airflow being measured because of the difference in capacity between various masks. A previous study reported that there were no statistically significant differences in CPF among various masks of different sizes [21]. However, there are no other reports on whe  
Figure 2: Cough peak flow (CPF) measurement devices and posture. (**a**) Spirometer. (**b**) Peak flow meter. (**c**) Face mask. (**d**) Infection prevention filter. (**e**) Conventional CPF measurement method.  
mask sizes for different participants can influence the cough airflow being measured. Thus, more methodologically rigorous studies are needed to definitively identify the effect of the mask size on CPF. Figure 2d shows an infection prevention filter. Although there are quite a few papers that do not describe the use of infection prevention filters in the measurement method, the filter should be used.  
The purpose and technique of CPF measurement should be explained to all participants before the measurement; moreover, how to perform the measurement [21] should be demonstrated. Participants are instructed to close the glottis after each insufflation and to coughing [27, 28]. After one or two trial attempts, participants are instructed to make a maximal effort and are closely watched to ensure no air leakage from the gap between the mask and face. When using a mouthpiece instead of the mask, a nose clip must be worn. To assess CPF, subjects perform a maximal cough effort after an as deep an inspiration as possible.