since blood pressure varies throughout the day, home measurements should be taken at the same time of day. a joint scientific statement from the american heart association, american society of hypertension, and preventive cardiovascular nurses association on home monitoring in 2008 recommended that 2 to 3 readings should be taken in the morning (after awakening, before washing/dressing, taking breakfast/drink or taking medication) and another 2 to 3 readings at night, each day over a period of 1 week. it was also recommended that the readings from the first day should be discarded and that a total of ≥12 readings (i.e. at least two readings per day for the remaining 6 days of the week) should be used for making clinical decisions. observer error there are many factors that can play a role in the blood pressure reading by physician, such as hearing problem, auditory perception of the physician. karimi hosseini et al. evaluated the interobserver differences among specialists without any auditory impairment, and reported 68% of observers recorded systolic blood pressure in a range of 9.4 mmhg, diastolic blood pressure in a range of 20.5 mmhg and mean blood pressure in a range of 16.1mmhg. neufeld et al. reported standard deviations for both systolic and diastolic readings were roughly 3.5 to 5.5 mm hg. in general standard deviation for the diastolic pressure would be greater because of the difficulty in judging when the sounds disappear. white-coat hypertension for some patients, blood pressure measurements taken in a doctor's office may not correctly characterize their typical blood pressure. in up to 25% of patients, the office measurement is higher than their typical blood pressure. this type of error is called white-coat hypertension (wch) and can result from anxiety related to an examination by a health care professional. white coat hypertension can also occur because, in a clinical setting, patients are seldom given the opportunity to rest for five minutes before blood pressure readings are taken. the misdiagnosis of hypertension for these patients can result in needless and possibly harmful medication. wch can be reduced (but not eliminated) with automated blood pressure measurements over 15 to 20 minutes in a quiet part of the office or clinic. in some cases a lower blood pressure reading occurs at the doctor's - this has been termed 'masked hypertension'. alternative settings, such as pharmacies, have been proposed as alternatives to office blood pressure. the threshold for blood pressure from pharmacy readings is 135/85 mmhg, suggesting a reduced white coat effect, similar to daytime ambulatory measurements. invasive arterial blood pressure is most accurately measured invasively through an arterial line. invasive arterial pressure measurement with intravascular cannulae involves direct measurement of arterial pressure by placing a cannula needle in an artery (usually radial, femoral, dorsalis pedis or brachial). the cannula is inserted either via palpation or with the use of ultrasound guidance. the cannula must be connected to a sterile, fluid-filled system, which is connected to an electronic pressure transducer. the advantage of this system is that pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed. this invasive technique is regularly employed in human and veterinary intensive care medicine, anesthesiology, and for research purposes. cannulation for invasive vascular pressure monitoring is infrequently associated with complications such as thrombosis, infection, and bleeding. patients with invasive arterial monitoring require very close supervision, as there is a danger of severe bleeding if the line becomes disconnected. it is generally reserved for patients where rapid variations in arterial pressure are anticipated. invasive vascular pressure monitors are pressure monitoring systems designed to acquire pressure information for display and processing. there are a variety of invasive vascular pressure monitors for trauma, critical care, and operating room applications. these include single pressure, dual pressure, and multi-parameter (i.e. pressure / temperature). the monitors can be used for measurement and follow-up of arterial, central venous, pulmonary arterial, left atrial, right atrial, femoral arterial, umbilical venous, umbilical arterial, and intracranial pressures.