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Holter monitor

A Holter monitor is a small, wearable, battery-powered (ECG) device that continuously records a person's heart rhythm and electrical activity for 24 to 48 hours or longer, enabling the detection of irregular heartbeats or arrhythmias that may not appear during a standard ECG. Developed in the late by American biophysicist Norman J. Holter and his team, the device originated from advancements in and , with the first practical version introduced in 1961 by Del Mar Engineering following collaborations with the Holter Research Foundation. Initially briefcase-sized and weighing under 5 pounds (2 kg), early models evolved rapidly into compact, portable units by the , with modern versions resembling a small and capable of for up to 14 days of . The primary purpose of a Holter monitor is to diagnose cardiac arrhythmias, correlate symptoms like or with abnormal rhythms, evaluate the effectiveness of heart medications or pacemakers, and assess risks in conditions such as syncope or cryptogenic . It is particularly useful for capturing intermittent issues that evade shorter tests, using 2 to 12 electrodes placed on the chest to detect variations in , including (SVT), (VT), and . Patients wear the device during normal daily activities, maintaining a of symptoms and events. The device records digitally for later by a . Over the decades, Holter has become a cornerstone of ambulatory , with ongoing innovations like AI-driven , extended-wear patches up to 30 days, and integration with loop recorders expanding its clinical applications (as of 2025).

Overview

Definition

A Holter monitor is a portable type of electrocardiography (AECG) device that continuously records the heart's electrical activity over extended periods, enabling the detection of cardiac rhythms during everyday activities. It functions by using electrodes placed on the chest to capture electrocardiographic (ECG) signals, which are then stored for later analysis, distinguishing it from stationary monitoring systems. Typically, a Holter monitor records 2 to 12 leads of ECG data, with most modern models using 2 to 3 leads for sufficient . The device is lightweight and compact, often weighing around 99 grams and measuring approximately 110 × 70 × 30 mm for standard configurations, making it suitable for unobtrusive wear. Recording durations generally span 24 to 48 hours, though advanced versions support up to 14 days of continuous monitoring with extended battery life and digital storage. In contrast to a standard ECG, which captures a brief snapshot of heart activity lasting only seconds to minutes, the Holter monitor provides prolonged, uninterrupted recording to identify transient or intermittent events that might otherwise go undetected. This continuous approach leverages the device's portability to track rhythms in real-world conditions.

Purpose and Indications

The primary purpose of a Holter monitor is to detect cardiac arrhythmias, myocardial ischemia, or other abnormalities that may not be evident during a standard resting electrocardiogram (ECG) by providing continuous ambulatory monitoring over 24 to 48 hours during a patient's normal daily activities. This extended recording captures transient events that occur sporadically, offering insights into heart rhythm variations under real-world conditions. Specific indications for Holter monitoring include evaluating symptoms such as , syncope, near-syncope, or to establish correlations with underlying rhythm disturbances. It is also used for following a , where it helps evaluate prognosis and the potential for sudden cardiac death by quantifying burden. Additionally, the device monitors the efficacy and safety of antiarrhythmic medications, detecting both therapeutic responses and proarrhythmic effects. In patients with , Holter monitoring screens for associated cardiac effects, such as arrhythmias exacerbated by nocturnal . To enhance diagnostic accuracy, Holter monitoring integrates with patient-maintained diaries, where individuals record the timing and nature of symptoms like , , or skipped beats, allowing clinicians to correlate these events directly with ECG tracings. Patients may also activate a marker on the device during symptom onset to timestamp ECG data for precise analysis. This symptom-rhythm linkage is crucial for guiding treatment decisions in settings.

History

Invention and Early Development

The development of the Holter monitor began in at the Holter Research Laboratory in , USA, where experimental physicists Norman J. Holter and Wilford R. "Bill" Glasscock initiated work on radio systems for biomedical applications. Their efforts were spurred by a suggestion from renowned cardiologist in the early 1950s, who emphasized the need for portable devices to capture cardiac electrical activity during everyday activities, shifting focus from stationary to ambulatory monitoring. Holter, a biophysicist with prior experience in non-invasive electrical measurements, collaborated with Glasscock to adapt principles originally explored in and contexts to medical diagnostics. The first functional prototype emerged in , marking a significant milestone in portable cardiac recording. This device employed technology for signal amplification and processing, paired with reel-to-reel for analog , allowing continuous electrocardiogram (ECG) capture without tethering patients to bulky . Weighing approximately 85 pounds in its earliest iterations, the backpack-sized system represented a leap toward portability, though it was still cumbersome and required improvements for practical clinical use. Initial testing on patients demonstrated its potential to detect arrhythmias during normal activities, validating the concept of long-term ambulatory monitoring. Commercialization followed swiftly in 1962, when Holter partnered with Del Mar Engineering Laboratories to produce the device under the Avionics Research Products Corporation. The initial models supported 8-12 hour recordings, constrained by the limitations of analog tape storage and the physical bulk of the components, which patients carried in a vest or . This release enabled widespread adoption in , providing physicians with insights into transient heart conditions that standard ECGs could miss. Early applications focused on outpatient settings, establishing the foundation for extended protocols.

Technological Advancements

The transition from analog tape-based recording to storage in Holter monitors began in the late and accelerated through the , replacing bulky reel-to-reel systems with compact solid-state memory that enabled device and extended recording durations of up to 14 days. This shift to formats, including integration, improved data fidelity, reduced mechanical failures, and facilitated automated analysis, marking a pivotal evolution in ECG . In the 2020s, advancements have focused on wireless and -based designs, such as the single-use UbiqVue biosensor launched in , which employs a wearable, dual-channel ECG for continuous integrated with cloud-based to streamline workflows and enable near access. (AI) has emerged as a key innovation for artifact reduction and automated analysis, with algorithms like those in the Zio ECG monitor achieving 99% accuracy in detecting arrhythmias while filtering noise from motion or poor contact, thus enhancing diagnostic reliability. Additionally, integration with electronic health records (EHRs) has advanced same-day capabilities, as seen in systems like ReactDx, which automate data transfer to platforms for rapid clinical review and reduced turnaround times. From 2023 to 2025, has driven the adoption of fully wearable form factors, transforming Holter monitors into lightweight, patches that prioritize patient comfort and compliance during extended wear. These developments have enabled prolonged monitoring periods, with Philips data demonstrating 2.5 times more actionable findings after 14 days compared to traditional 24-hour studies, significantly improving diagnostic yield in syncope and cases. The global Holter ECG market has grown to approximately $0.49 billion in 2025, propelled by the rise of wearables that incorporate and connectivity for broader applications.

Components

Recorder and Electrodes

The Holter monitor recorder is a compact, battery-powered device designed to capture continuous electrocardiographic (ECG) signals over extended periods, typically 24 to 48 hours or longer. It features built-in ECG amplifiers to amplify weak electrical signals from the heart, analog-to-digital (A/D) converters to digitize these signals for processing, and support for 2 to 12 leads to record cardiac activity from various angles, with 3 to 5 leads being standard for routine monitoring and up to 12 for more comprehensive assessments. The battery provides power for 80 to 100 hours of operation, enabling portability without frequent recharging. Many contemporary recorders integrate triaxial accelerometers, which detect motion in three dimensions to correlate ECG readings with levels, aiding in the interpretation of variations during daily routines. Over time, the recorder's design has evolved from bulky or briefcase-sized units in early models to modern pocket-sized versions, enhancing patient comfort and mobility. Electrodes serve as the interface for signal acquisition, consisting of disposable adhesive patches that adhere to the skin for stable contact. Standard configurations use 3 to 5 electrodes for basic monitoring, while up to 12 can be employed for a full 12-lead ECG; these are typically made with silver/silver-chloride (Ag/AgCl) sensing elements to ensure low impedance and accurate from the skin. Wireless electrode options, often integrated into patch systems, minimize cable tangling and improve wearability during extended use. Modern variants include patch-style recorders, which adhere directly to the chest like a large for enhanced comfort and reduced visibility under clothing; these Holter-specific devices weigh under 100 grams—such as 18 grams for some models—and feature water-resistant designs (e.g., IP67 rating) allowing patients to without removal.

and

The evolution of data storage in Holter monitors began with analog methods in the mid-20th century, where early devices from the late and early , such as prototypes developed by Holter and Del Mar Engineering, employed reel-to-reel tape recorders to capture single-lead ECG signals for several hours, limited by bulky designs weighing around 75 pounds and battery life. By the late , advancements such as those from Del Mar Avionics extended recording durations to 6-12 hours using miniaturized reel-to-reel tapes, while the saw the introduction of cassette-based systems, like frequency-modulated analog cassettes from Medical Systems, enabling up to of continuous monitoring. Earlier analog formats from the were constrained to 24-hour limits due to tape capacity and playback requirements, necessitating manual transcription for analysis, but cassettes overcame these to support up to . Contemporary Holter monitors have transitioned to digital storage, utilizing solid-state or cards to record multi-lead ECG data in standardized formats such as the European Data Format (EDF) or EDF+, which facilitate and efficient for long-term use. Storage capacities typically range from 512 MB to 4 GB or more, accommodating high-resolution recordings sampled at 250-500 Hz (or up to 1000 Hz in advanced models) for 14+ days, generating 40-100 MB of compressed data per 24-hour period across 2-3 channels. Many devices incorporate patient-activated event buttons that timestamp symptomatic episodes, embedding directly into the stored files for correlation with ECG waveforms during later review. Data transmission from Holter monitors occurs post-recording via USB interfaces for direct download to software on computers, ensuring reliable of raw EDF files without constraints in models. Wireless-enabled variants, approved since 2011, support cellular or uploads to secure cloud platforms, converting ECG data into encrypted streams for remote access by clinicians. In 2025 systems, these transmissions adhere to HIPAA-compliant protocols, including and access controls, to protect patient health information during and sharing.

Analysis Software

Analysis software for Holter monitors processes recorded electrocardiogram (ECG) data to identify cardiac events and generate clinical insights. These tools typically employ algorithms to scan ECG signals automatically, enabling efficient review by clinicians. Commercial platforms, such as those from and , integrate these functions into user-friendly interfaces that support data import from digital storage formats. Core functions include automated detection, which identifies heartbeat peaks in noisy ambulatory signals, often using models for accuracy in Holter datasets. classification follows, categorizing events like premature ventricular contractions () and (AFib) through methods such as template matching, where ECG waveforms are compared against predefined patterns. (HRV) analysis computes metrics like standard deviation of NN intervals (SDNN) for time-domain assessment of autonomic function and root mean square of successive differences () for short-term variability, providing quantitative measures of cardiac stability. Advanced features encompass artifact removal, with post-2020 AI-driven techniques enhancing signal quality by mitigating motion-induced noise in recordings. ST-segment detects ischemic episodes by measuring deviations from , supporting ischemia evaluation in long-term monitoring. detection identifies pacing spikes and assesses device performance, including failures in sensing or capture. These tools produce summary reports featuring graphs of trends, event histograms, and distributions for visual interpretation. User interfaces in platforms like Philips' Zymed and GE's CardioDay facilitate interactive review, allowing clinicians to edit detections and prioritize critical events. They support 12-lead ECG reconstruction from reduced-lead recordings, such as EASI configurations, to derive comprehensive views. Export options include PDF reports and integration with electronic health records (EHR) for seamless clinical workflow.

Procedure

Application and Setup

The application of a Holter monitor begins with thorough preparation to optimize and signal quality. The patient's chest area is cleaned with wipes to remove oils, sweat, and residues, while any lotions or moisturizers are avoided to prevent with . If is present in the electrode placement sites, it is shaved or clipped using a or clippers to ensure secure attachment and minimize artifact from poor contact. This preparation reduces the risk of signal noise and enhances recording accuracy during the monitoring period. Electrode placement follows a standardized configuration, typically using a 5-lead system for most Holter monitors to capture multiple ECG views. The right arm (RA) is positioned on the right upper chest near the , the left arm (LA) on the left upper chest near the , the right leg (RL) as a ground on the right lower , the left leg (LL) on the lower left near the , and the chest (V) lead in a precordial position such as modified V5 over the left fifth at the anterior axillary line. These positions, often based on the Mason-Likar modification, allow for continuous monitoring of leads I, II, III, and a precordial lead while accommodating movement. The , which are small adhesive patches with conductive gel, are firmly pressed onto the skin to establish good contact. Once electrodes are applied, the lead wires are connected to the portable recorder, a compact device about the size of a deck of cards that is secured via a or . A electrocardiogram (ECG) is recorded immediately after attachment to verify lead placement, assess signal quality, and establish a reference rhythm before the patient resumes normal activities. The technician instructs the patient on using the event button to mark episodes of symptoms such as or , which correlates ECG data with clinical events for later analysis. Monitoring duration is typically set for 24 to 48 hours, though it can extend to 72 hours or more depending on clinical indications. Safety considerations are integral to the setup process. Prior to application, the assesses for known allergies to adhesives or hydrogels in the electrodes, opting for alternatives if necessary to prevent irritation or . For patients with pacemakers or implantable cardioverter-defibrillators, the Holter monitor is compatible and does not interfere with device function, allowing simultaneous assessment of paced rhythms. No calibration beyond the baseline ECG is required, but the setup includes checks for loose connections or sources.

Patient Monitoring and Removal

Once the Holter monitor is applied, patients are instructed to continue their normal daily routines, including work and exercise, to capture heart activity under typical conditions and potentially provoke symptomatic events. This approach helps correlate any recorded arrhythmias with specific activities, though excessive movement may cause artifacts from loose electrodes or wire displacement, which can be noted in the patient's diary for later analysis. Patients must avoid water exposure to prevent damage to the device, as most Holter monitors are not waterproof; this includes refraining from showering, bathing, or swimming during the monitoring period, unless using a model with specific instructions for temporary disconnection. Additionally, patients should limit exposure to potential sources of , such as magnets, high-voltage areas, electric blankets, metal detectors, ovens, and keeping cell phones or music players at least 6 inches away from the monitor. To facilitate accurate interpretation, patients are required to maintain a detailed logging symptoms like , , , or , along with the exact time and associated activities, such as "lightheadedness during uphill walk at 2:15 p.m." After the prescribed monitoring duration, typically 24 to 48 hours, the device is removed either by clinic staff or by the patient under guidance, depending on the model. Removal involves gently peeling off the electrodes by hand—avoiding scissors or sharp tools to prevent injury—followed by cleaning the skin with soap and water to remove any adhesive residue. The patient then returns the monitor, wires, and to the healthcare provider for immediate data download and , ensuring no loss of recordings.

Data Analysis and Interpretation

Processing Methods

The processing of raw Holter monitor data begins with importing the electrocardiogram (ECG) signals from the device's storage medium into analysis software, where the data is retrieved automatically along with associated such as identification, recording start time, and duration. This step ensures seamless transfer for subsequent algorithmic handling without loss of temporal alignment. Following import, noise filtering is applied to enhance signal clarity; for instance, baseline wander caused by or electrode movement is removed using high-pass filters with frequencies around 0.5 to 0.6 Hz, which effectively suppress low-frequency artifacts while preserving QRS complexes. Beat detection then identifies QRS complexes through established algorithms like the Pan-Tompkins method, which combines bandpass filtering, differentiation, squaring, and moving window integration to reliably locate R-peaks in real-time ECG streams, achieving high sensitivity in settings. Once initial processing is complete, the ECG undergoes segmentation to facilitate targeted analyses. The recording is divided into fixed epochs, such as 5-minute intervals, totaling 288 segments over 24 hours, enabling calculations of (HRV) metrics like the standard deviation of normal-to-normal intervals (SDNN) within each quasi-stationary period. Events of potential clinical interest, including arrhythmias or ST-segment changes, are flagged algorithmically for manual verification by clinicians, with visual indicators such as colored lines overlaid on the to highlight segments requiring review. To optimize , especially for long-term recordings, compression techniques are employed, such as modified or wavelet-based methods, which reduce file sizes by exploiting ECG signal redundancy while maintaining diagnostic fidelity with minimal distortion. Quality assurance throughout processing involves evaluating signal integrity using metrics like the (SNR), which quantifies the proportion of useful QRS energy relative to background noise and helps identify segments suitable for analysis. Recent AI enhancements, integrated since 2023, leverage models to automate detection with accuracies exceeding 95% sensitivity for conditions like , surpassing traditional thresholds and reducing manual review burden in Holter workflows.

Clinical Reporting

The clinical reporting of Holter monitor data begins with the interpretation workflow, where automated processing algorithms initially flag potential abnormalities such as arrhythmias or ischemic events from the continuous ECG recording. A cardiologist then reviews these flags, verifying or adjusting them based on manual analysis of the full , while correlating flagged events with patient diary entries that document symptoms like , , or to establish clinical relevance. This synthesis ensures that reported findings reflect both objective ECG data and subjective patient experiences, guiding accurate diagnosis. Key outputs in Holter reports include quantitative metrics such as average , derived from R-R interval analysis, and arrhythmia burden, often expressed as the percentage of time spent in specific rhythms like , with higher burdens associated with increased clinical risks such as or hospitalization. Diagnostic statements provide clear summaries, such as "intermittent " or "," supported by representative ECG strips. Reports also incorporate risk stratification scores, like those assessing sudden cardiac death probability based on arrhythmia frequency, alongside recommendations for further testing such as or electrophysiological studies. By 2025, standards for Holter clinical reporting emphasize digital formats, leveraging cloud-based to generate actionable reports with interactive visualizations, including zoomable ECG waveforms and trend graphs for . These reports enable same-day turnaround and seamless integration into electronic health records (EHRs), facilitating multidisciplinary access for cardiologists, providers, and other specialists to support timely decision-making.

Clinical Applications

Diagnostic Uses

Holter monitoring plays a pivotal role in detecting paroxysmal arrhythmias that may not be captured during standard electrocardiograms, particularly in symptomatic patients experiencing intermittent symptoms such as or . It is commonly used to identify paroxysmal (AF), where short episodes can be documented over 24-48 hours, aiding in the of cryptogenic sources or efficacy . Similarly, it facilitates the of paroxysmal supraventricular tachycardia (SVT) by correlating ECG changes with patient-reported symptoms during daily activities. For , Holter monitoring identifies prolonged pauses or , especially in cases of syncope; pauses exceeding 3 seconds while awake may suggest underlying conduction issues. Additionally, it quantifies ectopic beats, such as ventricular premature complexes, in high-risk populations like post-myocardial infarction patients, providing insight into arrhythmic burden and risk. In ischemia assessment, Holter monitoring excels at capturing transient ST-segment changes that reflect myocardial ischemia during everyday activities or , often when exercise testing is inconclusive. These changes, typically greater than 0.1 mV lasting at least 1 minute, occur in approximately 80% of ischemic episodes, many of which are silent and exhibit circadian patterns with a morning peak. Post-myocardial (), it is employed for to detect reperfusion arrhythmias or residual ischemia, with silent ischemic events observed in 30-43% of patients within 30 days, serving as a prognostic marker for future cardiac events including . Beyond arrhythmias and ischemia, Holter monitoring evaluates conduction abnormalities, such as or bundle branch disturbances, by documenting intermittent high-grade blocks or pauses that correlate with symptoms like or syncope. For drug safety, it assesses prolongation induced by medications, measuring QTc intervals over extended periods to identify risks of , though caution is advised due to variability in Holter-derived measurements compared to standard ECGs. In sleep studies, it captures nocturnal events like bradycardia-tachycardia cycles associated with or increased ventricular ectopy during apneic episodes, guiding further when cyclic heart rate variations are noted.

Therapeutic Monitoring

Holter monitors play a crucial role in therapeutic monitoring by providing continuous ambulatory ECG data to evaluate the effectiveness of interventions and guide adjustments in patient management. In assessing drug , particularly for antiarrhythmic agents, Holter recordings establish a baseline frequency of s such as premature ventricular complexes () and nonsustained ventricular tachycardia (NSVT), allowing quantification of response through metrics like a ≥75% reduction in or ≥90% suppression of NSVT episodes post-medication. For example, serial Holter monitoring before and after initiating can confirm suppression rates, with studies showing reproducible predictions of in up to 83% of cases when repeat recordings verify consistent reduction. This approach helps differentiate true therapeutic effects from spontaneous variability in burden. Beyond efficacy, Holter is essential for detecting proarrhythmic effects of antiarrhythmic drugs, which can manifest as new-onset , , or aggravated preexisting arrhythmias. By comparing pre- and post-treatment recordings, clinicians identify increases in ventricular ectopy or NSVT that signal potential harm, enabling timely discontinuation or dose adjustment. Such is particularly valuable in high-risk patients with ischemic heart disease, where proarrhythmic responses have been linked to worse outcomes during follow-up. In device follow-up for pacemakers and implantable cardioverter-defibrillators (ICDs), Holter monitors assess pacing capture thresholds and overall function to optimize programming. For pacemakers, 24-hour recordings verify automatic threshold tracking algorithms, confirming backup pacing activation during loss of capture and ensuring stable evoked responses over time, which supports adjustments to output voltage or for and . Post-interventional monitoring after procedures like (AFib) ablation relies on Holter recordings to track recovery and detect early recurrences, informing decisions on anticoagulation or repeat . A 48-hour Holter typically captures 67% of AFib episodes in the first few months post-procedure, providing metrics on recurrence rates that guide therapeutic escalation if burden exceeds predefined thresholds. This short-term surveillance, often at 3-6 months, complements longer-term implantable monitoring by establishing immediate post-ablation patterns.

Advantages and Limitations

Benefits

The Holter monitor's primary benefit lies in its portability, which permits to perform unrestricted daily activities while the device records cardiac electrical activity. This capability is crucial for capturing rare or intermittent cardiac events, such as arrhythmias, that may only manifest during specific physical exertions, , or —conditions not replicable in a controlled clinical environment with stationary ECGs. Unlike traditional in-office ECGs that limit patient movement and provide only data, the Holter's (typically the of a small ) enables continuous monitoring in real-world settings, often over 24 to 48 hours, without significantly impeding lifestyle. Continuous recording distinguishes the Holter monitor by delivering a comprehensive that includes every heartbeat, offering vital temporal context for diagnosing disturbances. For instance, integration with a patient-maintained activity and symptom allows clinicians to correlate arrhythmic episodes with triggers like exercise or changes, revealing patterns such as activity-triggered arrhythmias that intermittent devices might overlook. This approach yields higher diagnostic utility than event monitors for short-term evaluations of frequent symptoms, as it eliminates reliance on patient-initiated recordings and ensures no events are missed during the monitoring period. Modern wireless designs amplify this advantage by streamlining wearability and reducing patient discomfort. As a non-invasive, outpatient , Holter proves cost-effective for initial assessment, avoiding the expenses and risks associated with inpatient evaluations or more advanced . It achieves a diagnostic yield of 33% to 35% in patients presenting with , providing actionable insights into and rhythm abnormalities with high reliability when correlated to symptoms. As of 2025, integrations of (AI) and (ML) in analysis platforms have further enhanced efficiency, with market analyses projecting AI-enabled solutions to drive early detection and processing accelerations beyond prior benchmarks of over 25%, supporting broader outpatient diagnostics.

Drawbacks and Alternatives

Despite its utility in short-term cardiac , the Holter monitor has several drawbacks that can impact patient experience and diagnostic reliability. Adhesives used to secure electrodes often cause mild or allergic reactions in some patients, potentially leading to discomfort or early removal of the device. Additionally, the monitor's typical recording duration of 24 to 48 hours limits its effectiveness for detecting infrequent arrhythmias, in contrast to implantable devices that provide for years. Motion artifacts from daily activities frequently degrade signal quality, necessitating extensive manual review by clinicians to distinguish true cardiac events from . Several alternatives address these limitations by offering extended monitoring periods, improved comfort, or targeted recording capabilities. Event monitors are symptom-activated devices that patients wear or carry for weeks, recording ECG data only during reported symptoms or automatically detected events, making them suitable for intermittent issues. Patch monitors, such as the Zio XT, adhere directly to the skin without leads and provide continuous recording for up to 14 days, reducing wire-related discomfort and motion artifacts. Consumer wearables like the Apple Watch offer on-demand ECG spot checks via single-lead recordings, ideal for preliminary screening but not continuous long-term use. For prolonged needs, implantable loop recorders are surgically placed under the skin and can monitor for up to three years, automatically detecting and storing arrhythmia data. As of 2025, advancements in wearable Holter devices and mobile health technologies, including real-time cloud-based analytics and wireless patches like ePatch, further expand options for extended, patient-friendly monitoring with reduced artifacts. Trade-offs between Holter monitors and alternatives highlight the balance between continuous short-term data capture and extended comfort. While Holter excels in providing uninterrupted 24- to 48-hour recordings for immediate diagnostic insights, patch monitors like the Zio offer superior patient comfort and detect clinically actionable arrhythmias at nearly twice the rate of traditional Holter monitoring, according to 2023 studies comparing 7-day patches to 24-hour Holters (34.5% vs. 19.0% detection rates).