혈소판 수 측정시의 문제점들

백인기,김미애 인제대학교 1999 仁濟醫學 Vol.20 No.1

혈소판 수 측정 시에 여러 가지 오차가 발생할 수 있다. 그 중 혈소판 수 감소와 증가가 문제될 수 있으며 특히 가성 혈소판 수 감소 및 증가 요인들을 파악하여 보았다. Platelet count is important to evaluate the hemostatic system and abnormality in platelet count can lead to bleeding complication in patients. Errors in platelet count can be occurred sometimes and many clinicians require more accurate platelet counting. We reviewed principles of platelet counting and also reviewed causes of common errors in platelet counting. 1) Automated blood cell counters are used for routine platelet counting in most laboratory. Principles of automated blood cell counting is two, the one is electrical impedance and the other is light scattering. Small difference in platelet count can be occurred between two types of blood cell counter. 2) Manual platelet counting is not performed in routine platelet count, but if platelet number would be decreased to less than 50.0×109/L in automated blood cell counting, hemocytometer counting by phase contrast microscope or platelet counting in peripheral blood smear is recommended to confirm decreased platelet count. 3) True thrombocytopenia may be the result of either marrow production problem or peripheral destructive process. 4) Causes of pseudothrombocytopenia are shown as follows ㆍ platelet clumps due to improper mixing of EDTA contained CBC bottle. ㆍ EDTA induced pseudothrombocytopenia ㆍ platelet satellitosis ㆍ giant platelets 5) True thrombocytosis may be result of either reactive process or chronic myeloproliferative disorders.

유아의 수세기 능력과 수세기에 대한 인식

서동미,윤은미,문주형 한국영유아교원교육학회 2005 유아교육학논집 Vol.9 No.2

The purpose of this study was to provide basic data of children's number counting through analyses on number counting of children and determine, their ideas about it. It also aimed to determine the relation between number counting of children and their ideas about it. The subjects were 245 children from three to six-year-old age groups. The study results were as follows: First, We identified that older children showed better demonstration than younger children in oral counting and object counting. Second, With regards to children's reasons for counting, as the age increased, the children's ideas changed from figuring out the number for themselves as an activity to recognizing how many numbers they actually count. Also, With regards to what children counted, most children counted numbers as parts of an object-related activity. 본 연구는 어린이집과 유치원의 만 3,4,5,6세 유아 245명을 대상으로 유아의 연령에 따른 수세기 능력과 수세기에 대한 인식을 알아봄으로써 유아교육 현장에서의 수세기 교육의 기초 자료를 제시하고자 하였다. 연구 결과 유아의 연령이 증가되면서 유아의 말로 수세기와 사물 수세기의 능력이 높아짐을 알 수 있었다. 유아의 수세기에 대한 인식은 연령이 증가될수록 나 자신을 위한 수세기에서 몇 개인지 알기 위한 수세기로 변화되었다. 또한 수세기의 대상에 대해 모든 연령의 유아들이 물건과 관련된 활동으로서의 수세기를 인식하고 있었다.

List Mode에서 PET/CT Scanner의 직선성 평가

최현준,김병진,이또 미키코,이홍재,김진의,김현주,이재성,이동수,Choi, Hyun-Jun,Kim, Byung-Jin,Ito, Mikiko,Lee, Hong-Jae,Kim, Jin-Ui,Kim, Hyun-Joo,Lee, Jae-Sung,Lee, Dong-Soo 대한핵의학기술학회 2012 핵의학 기술 Vol.16 No.1

Rb-82를 이용한 PET 검사는 심근 관류의 임상적 평가에 중요한 역할을 한다고 알려져 있다. 그러나 PET/CT의 dead time 때문에 count value와 방사능농도 사이의 직선성이 유지되지 않는다면 데이터를 획득할 때 좌심실 입력 함수가 과대평가되어 심근관류가 과대평가될 수 있다. 본 연구에서는 리스트 모드에서 획득한 PET data에서 방사능농도에 따른 count value의 직선성을 평가하였다. Biograph 40 True Point PET/CT를 이용하여 직경 12 cm, 길이 10.5 cm의 cylindrical phantom에 F-18 333 MBq과 물 800 mL를 채우고 7반감기동안 10min frame/bed로 리스트 모드를 이용하여 획득하였다. Raw data는 OSEM (order: 4, subsets: 8)과 FBP (Gaussian filter FWHM 5 mm) 알고리즘을 이용하여 재구성하였다. Sinogram 정보에서 prompt counts, net true counts, random counts를 측정하였다. 재구성 된 phantom 영상에 ROI를 설정하여 총 계수와 background를 측정하고 background correction을 사용하여 count value를 측정하여 직선성을 평가하였다. 리스트 모드를 이용하여 sinogram에서 측정된 prompt counts는 방사능농도에 비례하여 증가하였다. 낮은 방사능농도에서 net true counts와 random counts는 방사능농도에 따라 증가하였다. 높은 방사능농도에서는 net true counts의 증가율이 점차 감소되었고, 반면에 random counts의 증가율은 증가하였다. 그리고 OSEM과 FBP 알고리즘으로 재구성된 영상에서 측정한 count value의 차이는 없었고 방사능농도에 비례하여 count value가 증가하고 직선성이 유지되었다. Biograph 40 True Point PET/CT scanner는 재구성된 영상에서 낮은 방사능농도뿐만 아니라 높은 방사능농도(~416.25 kBq/mL)에서도 측정 된 count value와 방사능농도 사이의 직선성이 유지되는 것을 보여주었다. 따라서 실험에서 사용한 PET/CT scanner는 Rb-82, N-13, O-15, F-18을 이용한 heart dynamic PET study에서데이터의 정량적 분석에 유용할 것으로 판단된다. Purpose: Quantification of myocardial blood flow (MBF) using dynamic PET imaging has the potential to assess coronary artery disease. Rb-82 plays a key role in the clinical assessment of myocardial perfusion using PET. However, MBF could be overestimated due to the underestimation of left ventricular input function in the beginning of the acquisition when the scanner has non-linearity between count rate and activity concentration due to the scanner dead-time. Therefore, in this study, we evaluated the count rate linearity as a function of the activity concentration in PET data acquired in list mode. Materials & methods: A cylindrical phantom (diameter, 12 cm length, 10.5 cm) filled with 296 MBq F-18 solution and 800 mL of water was used to estimate the linearity of the Biograph 40 True Point PET/CT scanner. PET data was acquired with 10 min per frame of 1 bed duration in list mode for different activity concentration levels in 7 half-lives. The images were reconstructed by OSEM and FBP algorithms. Prompt, net true and random counts of PET data according to the activity concentration were measured. Total and background counts were measured by drawing ROI on the phantom images and linearity was measured using background correction. Results: The prompt count rates in list mode were linearly increased proportionally to the activity concentration. At a low activity concentration (<30 kBq/mL), the prompt net true and random count rates were increased with the activity concentration. At a high activity concentration (>30 kBq/mL), the increasing rate of the prompt net true rates was slightly decreased while the increasing rate of random counts was increased. There was no difference in the image intensity linearity between OSEM and FBP algorithms. Conclusion: The Biograph 40 True Point PET/CT scanner showed good linearity of count rate even at a high activity concentration (~370 kBq/mL).The result indicates that the scanner is useful for the quantitative analysis of data in heart dynamic studies using Rb-82, N-13, O-15 and F-18.

Machine Learning Approach for Simultaneous Determination of Counting Efficiencies of Strontium-90 and Strontium-89

Wook Sohn,Kahee Jeong,Ji-eun Park,Sooyeon Lim 한국방사성폐기물학회 2022 한국방사성폐기물학회 학술논문요약집 Vol.20 No.1

Radioactivity of radiostrontiums, Sr-89 and Sr-90, which are both pure beta-emitters, are generally measured via Cherenkov counting. However, the determination of Cherenkov counting efficiencies of radiostrontiums requires a complicated procedure due to the presence of Y-90 (also a pure betaemitter) which is the daughter nuclide of Sr-90. In this study, we have developed a machine learning approach using a linear regression model which allows an easier and simultaneous determination of the Cherenkov counting efficiencies of the radiostrontiums. The linear regression model was employed because total net Cherenkov count (Ct) from the three beta-emitters at time t after the separation of Y- 90, can be expressed as a linear combination of their respective time-varying radioactivities with their respective coefficients (parameters) being their counting efficiencies: Ct = εSr-90[ASr-90·exp(–λSr-90·t)] + εSr-89[ASr-89·exp(–λSr-89·t)] + εY-90[ASr-90·exp(1–λSr-90·t)], where ε is a counting efficiency, A is an initial activity, λ is a decay constant and t is time after the separation of Y-90, Thus, if we train the model with multiple Cherenkov counts measured from the three beta emitters, then we can obtain their estimates for counting efficiencies (so-called parameters) straightforward. For this, the model has been trained by two methods: Ordinary Least Squares (OLS) and Bayesian linear regression (BLR), for which two software packages, PyMC3 and Stan were employed to compare their performances. The results showed that the accuracy of the OLS was worse than that of the BLR. Particularly, the counting efficiency of Sr-90 was estimated to be smaller than 0, which is an unrealistic value. On the other hand, the estimates of the BLR gave realistic values which are close to the true values. Additionally, the BLR was able to provide a distribution for each counting efficiency (so-called “posterior”) from which various types of inference can be made including median and credible interval in the Bayesian statistics which is analogous to, but different from confidence interval in the Frequentist statistics. In the results of the BLR, the Stan package gave more accurate estimates than the PyMC3 package. Therefore, it is expected that counting efficiencies of the radiostrontiums including radioyttrium can be determined at the same time, more easily and accurately, by using the BLR with the Stan package and that the activities of radiostrontium also can be determined more easily by using the BLR if we know their counting efficiencies in advance. It is worth noting that the usage of the linear regression model in this study was different from the usual one where the trained model is used to predict a response value (count) from a set of unseen regressor values (activities).

소아 간질 환자에서 valproate 투여 후 혈소판 수의 변화

위주희,김영미,남상욱 대한소아청소년과학회 2009 Clinical and Experimental Pediatrics (CEP) Vol.52 No.1

Purpose : In adults, valproate (VPA) has been reported to be associated with thrombocytopenia. However, few studies have investigated this association in children, and the factors affecting platelet counts remain controversial. This study was undertaken to investigate changes in platelet counts following VPA therapy and related factors in children with epilepsy. Methods : Our subjects comprised 75 epileptic children who had received VPA monotherapy for at least 1 year between January 2000 and December 2005. We regularly examined platelet counts at intervals of 6 to 12 months after baseline platelet counts before VPA treatment. We analyzed changes in platelet counts according to several factors such as sex, seizure type, underlying causes of epilepsy, age at the start of medication as well as at the last follow up, duration of VPA administration, VPA dose, serum VPA level, and body weight at the last follow up. Results : Overall, the platelet counts decreased significantly after VPA therapy. The average baseline platelet count was 312.5±109.1×106/mL. At the last follow up, the average platelet count was 261.7±72.2×106/mL. We observed the relationship between platelet counts and serum levels of VPA; while children with serum VPA <75 µg/mL showed no change in platelet counts (P=0.102), children with serum VPA >75 µg/mL showed a significant decrease in platelet counts. Conclusion : VPA may cause a significantly decreased platelet count in children with epilepsy. High serum VPA levels were associated with development of thrombocytopenia. 목 적 : 소아 간질 환자에서 VPA 투여가 혈소판 수의 변화에 미치는 영향과 이에 관련된 인자들을 알아보고자 본 연구를 시행하였다. 방 법 : 2000년 1월부터 2005년 12월 사이에 부산대학교병원 소아과에서 간질로 진단 받고 VPA 단독 투여를 1년 이상한 환자 448명 중에서 6-12개월 간격으로 정기적인 혈액검사를 시행한 환아 75명을 대상으로 하였다. 성별, 경련의 형태, 간질의 원인이 되는 기저 질환의 유무, VPA 투여 전 연령, 최종 추적 시 연령, 투약 용량, 투약 기간, 혈중 VPA 농도, 투여 전 체중에 따른 두 군으로 분류하여 투여 전과 투여 후 가장 최근에 검사한 혈소판 수의 변화를 비교, 분석하였다. 결 과 : 전체 대상 환자 75명 중 54(72.0%)명이 투여 전에 비해 투여 후 최종 추적 시 혈소판 수가 통계적으로 유의하게 감소하였다(P=0.0001). 성별, 경련의 형태, 간질의 원인, VPA 투여 시작 연령, 최종 추적 시 연령, 투약 용량, 투약 기간, 체중을 기준으로 나눈 여러 군에서 VPA 투여 전과 투여 후 최종 추적 시 혈소판 수는 모두 통계적으로 의미 있는 감소를 보였다(P<0.05). 혈중 VPA 농도 75 µg/mL 이하인 환자군(P=0.102)에서는 치료 전과 치료 후 최종 추적 시 혈소판 수 비교에서 의미 있는 감소를 보이지 않은 반면 75 µg/mL 이상인 환자군(P=0.0000003)에서는 의미 있는 감소를 보였다. 또한 혈중 VPA 농도가 높을수록 혈소판 감소가 더 현저히 관찰되었다(r=-0.4). 결 론 : VPA 치료는 유의한 혈소판 수의 저하를 일으키나 심한 혈소판 저하증의 빈도는 낮으며, 임상 증상을 초래하는 경우는 드물다. VPA 투여 후 혈소판 감소 발생에 관계된 예측 인자로 혈중 VPA 농도가 가장 중요하므로 수술 예정이거나 혈소판 감소를 유발할 수 있는 다른 약물을 병용하는 경우, 고농도의 VPA 투여가 필요한 환자에 있어서는 주기적인 혈중 농도 검사가 권장된다.

소아 간질 환아에서 valproate 투여 후 혈소판 수 변화

위주희,남상욱 부산대학교 병원 암연구소 2007 부산대병원학술지 Vol.- No.21

Purpose: It has been known that valproate (VPA) therapy is associated with thrombocytopenia. But few studies have been conducted in children and there are conflicting results about the factors which affect the changes of platelet counts. This study was undertaken to investigate the changes of platelet counts after VPA therapy and the risk factors of platelet count changes in children taking VPA with epilepsy. Subject and Methods: Subjects were 75 children with epilepsy who had received VPA monotherapy over a period of2 months from January 2000 to December 2005. We checked platelet counts before VPA therapy and after VPA therapy for 2months and analyzed the relationship between changes in platelet counts and other factors such as sex, seizure type, underlying causes of epilepsy, age, VPA dose, serum VPA levels, and body weight. Results: In total population, the platelet counts were not changed significantly at 2 months after VPA therapy. The initial platelet count was 312.5± 109.1 X 10^(6)/mL. At 2months after VPA therapy, platelet count was 294.6±94.7x 10^(6)/ mL(P=0.054). The relationship between the platelet count decrease and other factors such as age and blood levels of VPA was observed in our study. In children older than 6 years old, the platelet counts before and two months after VPA theray were 295.1±85.4 X 10^(6)/mL, 275.6±94.3x 10^(6)/mL(P=0.027). In children who had serum VPA level higher than 75 μg/mL, the platelet counts before and two months after VPA theray were 324.1 X 102.4 X 10^(6)/mL, 297.4 ± 105.7 X 106/mL(p=0.028). The platelet count change was not affected by the other factors such as sex, seizure type, underlying causes of epilepsy, VPA dose, and body weight. Conclusions: The platelet counts don’t show any change 2 months after VPA therapy. But Older age and high blood level of VPA are risk factors of the decrease of platelet counts. Regular monitoring of serum VPA level and platelet counts is recommended especially in these group.

Accuracy of swimming wearable watches for estimating energy expenditure

( Mihyun Lee ),( Hyojin Lee ),( Saejong Park ) 한국스포츠정책과학원(구 한국스포츠개발원) 2018 International Journal of Applied Sports Sciences Vol.30 No.1

With the recent installation of waterproof function on wearable watches, various sports activities including walking, running and even swimming are monitored. Commercially available swimming wearable watches automatically identified stroke type, swim distance, stroke counts and energy expenditure (EE). Although the accuracy of estimating EE of walking, bilking and activities of daily life on activity monitors have been evaluated, it has not been examined for swimming. Thus, the purpose of the study was to evaluate the accuracy of estimating EE for swimming wearable watches (Apple Watch S2, Apple and Garmin Finex 3HR, Garmin). A total of 78 swimmers aged 20-59 years (female: 48%) participated in the study. All the participants wore Apple and Garmin and completed a set of swimming protocol comprising various speeds (0.4, 0.6, 0.8, 1.0, 1.2 m/s). At each swimming speed they were asked to swim for four minutes. Lap counts, stroke counts and energy expenditure (EE) from the Apple and Garmin were evaluated with the criterion measures. Lap counts and stroke counts were directly counted by the research assistant. The portable respiratory gas analyzer (K4b2, Cosmed, Italy) and a swimming snorkel (Aqua Trainer Snorkel, Cosmed, Italy) was used as the criterion measure of EE. The mean absolute percentage error (MAPE) of lap counting and stroke counts at various swimming speed were within 10% for Apple (lap counts: 0.5-6.1%, stroke counts: 6.2-9.3%) and about 20% for Garmin (lap counts: 0-20.6%, stroke counts: 6.8-17.6%). However, the MAPE of EE was higher for Apple (17.1%-151.7%) than for Garmin (17.9%-32.7%). The accuracy of estimating EE tended to improve with increasing swimming speed for both Apple and Garmin. The EEs from Apple were outside the equivalence zone except for at 1.2 m/s and were overestimated compared to the criteria. On the other hand, EEs from Garmin were within the equivalence zone at all speeds except for 1.2 m/s. In conclusion, Apple and Garmin wearable watches accurately measure lap counts and stroke counts. However, the accuracy of estimating EE are poor at slow to medium swimming speed. Further improvement is needed to estimate energy expenditure of swimming at various speed.

White blood cell differential counts in severely leukopenic samples: a comparative analysis of different solutions available in modern laboratory hematology

김아현,이원배,김명신,김용구,한경자 대한혈액학회 2014 Blood Research Vol.49 No.2

Background We evaluated the efficacy of white blood cell (WBC) differential counts in severely leukopenic samples by the Hematoflow method and by automated hematology analyzers and compared the results with manual counts. Methods EDTA-anticoagulated blood samples (175 samples) with WBC counts of 40‒990/μL were selected. Hematoflow differential counts were performed in duplicates employing flow cytometry using the CytoDiff reagent and analysis software. Differential counts were also performed using the DxH 800 (Beckman Coulter) and XE-2100 (Sysmex) automated hematology analyzers. The sum of the manual counts by a hematology technician and a resident were used as the manual counts. Results The total analysis time and hands-on time required by the Hematoflow method were shorter than those required by manual counting. Hematoflow counts were reproducible, showed a good correlation with automated analyzers, and also showed strong correlation with manual counts (r > 0.8) in neutrophils, lymphocytes, and monocytes. None of the cases containing less than 4% blasts as analyzed by the Hematoflow method had blasts in the manual counts, but 8 cases of 21 cases (38.1%) with over 4% blasts by Hematoflow had blasts in manual counts. Conclusion Hematoflow counts of severely leukopenic samples were reproducible and showed a good correlation with manual counts in terms of neutrophil, lymphocyte, and monocyte counts. The Hematoflow method also detected the presence of blasts. Manual slide review is recommended when over 4% blasts are found by Hematoflow.

인조광물섬유에 대한 NIOSH 7400 방법의 A 및 B 계수규칙 비교

신용철 ( Yong Chul Shin ) 한국산업위생학회 2006 한국산업보건학회지 Vol.16 No.1

There are many counting rules for analyzing man-made mineral fibers. The representatives are the NIOSH Method 7400 A and B counting rules. The two rules have different rules of length-to-width ratio(aspect ratio) and diameter. The A rule counts only fibers>5 μm in length, and only fibers with aspect ration>3:1. The B rule counts only ends of fibers>5 μm in length and <3 μm in diameter, and only fibers with aspect ratio ≥5:1. The A counting rule had been used before the B counting rule was introduced. The purpose of this study is to compare the A and B counting rules for airborne fibers from various man-made mineral fibers(glass wool fibers, rock wool fibers, refractory ceramic fibers, and continuous filament glass fibers) industries. There were significantly differences between the paired counts of A and B rules in all types of fibers(p<0.05). A rule counts/B rule counts(A/B ratios) were 1.52 for glass fibers, 1.53 for rock wool fibers, 1.19 for RCF, and 1.82 for continuous filament glass fibers. The counting results by A and B counting rules were highly correlated in glass wool fibers, rock wool fibers and refractory ceramic fibers(RCF) samples (r=0.96 for all types of fibers) except continuous filament glass fibers(r=0.82). Regression equations to correct for the differences between counting rules were presented in this paper.

요추천자 시술중 뇌척수압을 간단히 측정하는 방법

박승옥(Seung Ok Park),박원(Weon Park),김도연(Do Yeon Kim),허남진(Nam Jin Huh),최광해(Kwang Hae Choi),정철주(Chul Zoo Jung) 대한소아신경학회 1997 대한소아신경학회지 Vol.5 No.1

연구배경 : 소아에서 뇌척수압은 정확한 측정이 필요한 경우는 드물지만 압력이 정상인지 증가 되었는지를 아는 것은 중요할 수 있다. 요추천자 시술 중 협조가 되지 않은 소아에게 압력계를 이용해 뇌척수압을 측정하는 것이 쉽지 않다. 이에 저자들은, 소아에서 흔히 사용되는 척수천자침으로, 뇌척수액이 떨어지는 방울 수와 시간을 뇌척수압과 연관시키는 counting period를 결정함으로써, 뇌척수업을 측정할 수 있는 간단한 방법을 발견해 보고자 본 연구를 시행하였다. 방법 : 1997년 4월부터 5월까지 본원 소아과에 입원한 환자 중 뇌막 자극증상이나 패혈증으로 뇌척수액검사를 필요로 하는 15명을 대상으로, 소아의 척수천자 시술에 가장 많이 사용되는 22-guage 3.5-inch와 23guage 3.5-inch 척수천자침을 척수강내에 삽입한 후, 먼저 압력계로 척수액 방울 수를 1분 동안 센 다음, 상호 관계를 비교해 counting period를 결정하였다. Counting period가 결정된 다음에는, 전형적으로 10명의 환아에서 먼저 counting period동안 떨어지는 뇌척수액 방울수를 뇌척수압으로 결정하고, 압력계를 이용한 뇌척수압과 비교하였다. 결과 : 22 G 3.5inch 천자침을 이용한 counting period는 40초. 23 G 3.5-inch 천자침을 이용한 coungting period는 54초였다. 일정한 시간 동안에 천자침을 통해 떨어지는 뇌척수액 방울 수를 세어서, 그것을 뇌척수압으로 결정하는 counting period를 적용해, 전향적으로 10명의 환아에서 뇌척수압을 먼저 정한 후, 다시 압력계로 측정해 본 결과와 비교해 보았더니 모든 환아에서 10% 범위 내에 있었다. 결론 : 소아에서 뇌척수압을 측정하는 방법으로 counting period가 연구되었고 전향적으로 몇 명의 환자에서 시험되어졌다. 뇌척수액을 시험관에 받으면서 counting period동안 떨어지는 척수액 방울 수를 세기 때문에, 압력계로 측정하는 것보다 시술이 간단하고, 편리하며, 압력계를 파손할 염려가 없고 비용을 줄일 수 있으며 시술 중 오염의 빈도를 줄일 수 있을 것으로 사료된다. Background: Precise measurement of cerebrospinal(CSF) pressure rarely is needed, but it is frequently helpful to know whether CSF pressure is normal or elevated. It is often difficult to measure CSF pressure by manometer during lumbar puncture in uncooperated children. The purpose of this study was to determine appropriate counting periods for two commonly used spinal needles. Methods: Fifteen infant and children(from 1½ months to 9 years of age) with sepsis or meningeal irritation sign between April 1997 and May 1997 had measured CSF pressure. In this study, No. 22 or 23-guage, 3.5-inch, sharp, beveled spinal needle with a properly fitting stylet were used. After spinal needle is inserted in the lumbar subarachnoid space, estimated CSF pressure by manometer, and then counted drops of CSF during one minute period. A counting period can be dertermined such that the unmber of drops counted will approximate the CSF pressure in centimeters of H₂O. After these times were calculated, the method was verified in patients by comparing manometric pressure to drop count. Results: Counting periods for which the number of drops equals the pressure were then established for each needle size. Counting periods were determined at 40 seconds for 22-guage 3.5-inch needles, and 54 seconds for 23-guage 3.5-inch needles. The method was prospectively tested in 10 patients by comparing drop count over the calculated counting period to manometric pressures. In all cases, drop counts were within 10% of manometric pressure. Conculsions: This conting period method enables simple, rapid estimation of CSF pressure during lumbar puncture. Because drops can be counted while fluid is collected, the technique is more convenient than manometric pressue measures. Elimination of the manometer also reduced costs and may reduce the frequency of breaks in sterile technique.