Background Information




Q. Where can I measure?
A. Measurements can be performed just as easily at the bedside as during the regular patient consultation with the physician.

Q. When should I measure?
A. BCM measurements should be performed whenever the fluid status and body composition of a patient needs to be determined, e.g. in case of
  • change to the medication or other clinical intervention 
  • the occurrence of symptoms, 
  • perceived changes in body composition/lifestyle 
  • hospitalization

Measurements for hemodialysis patients should be performed pre-treatment and before cannulation. Post dialysis measurements may be obtained after 30 minutes of fluid re-equilibration. Peritoneal dialysis patients can be measured at any time.

Q. How often can I measure?
A. You can measure as often as you wish! There is no limitation; although dependent on patient the following is suggested:
  • CKD 3 – 4 : Quarterly 
  • HD and PD: monthly or more often in case if needed 
  • Acute patients: daily

Q. How long does a measurement take?
A. After the measurement and the subsequent calculations, results are displayed after just 2 min.

Q. Can my patient sit during measurement?
A. Measurements are best performed with patient in a relaxed, supine position. The patient may also sit during measurement, but feet should be raised in order to achieve an equal distribution of fluids. It is advisable to measure the same patient always in the same position to achieve the highest degree of reproducibility.

Q. How can I improve measurement quality?
  • Ensure patient is relaxed in the supine position 
  • Ensure skin is clean and good electrode contact has been achieved. Apply skin alcohol if needed 
  • Check correct electrode placement 
  • Check correct connection of patient lead 
  • Try to minimize sources of electrical disturbances (power sources, mobile phones etc.) 
  • Ensure legs are apart and arms are not in contact with the trunk.

Q. What is the impact of entering the wrong height? What is the effect of nude weight and clothed weight?


Q. What is the difference between single frequency bioimpedance analysis (SF-BIA), multi frequency BIA (MF-BIA) and bioimpedance spectroscopy (BIS)?
A. Most single frequency devices (SF-BIA) (and fat scales) measure only at 50 kHz and therefore measure impedance which is influenced by a combination of ECW and ICW.
There are multi frequency devices (MF-BIA) that measure from 3 to 8 frequencies (between 5-800 kHz), but these devices allow also no accurate differentiation between ECW and ICW. MF-BIA uses empirical linear regression models to evaluate FFM, TBW, ICW and ECW that usually only apply to healthy subjects.
Bioimpedance spectroscopy (BIS) uses physiological modeling and mixture equations (Cole–Cole plot and Hanai formulae) to first determine the electrical resistance of ECW and ICW and then calculate the volumes of these respective compartments. This is essential for identification of OH – the BCM – Body Composition Monitor uses the BIS technique.

Q. What are reactance, resistance and impedance?
A. Impedance defines the opposition to an alternating electrical current. Impedance is comprised of both resistance and reactance. By analyzing the resistance and reactance of human tissue certain properties of the tissue can be obtained. The resistance of the tissues to the electrical current determines the opposition of the tissue to the flow of electrons (dissipative loss). It is related to the amount of water present in tissues. The reactance by contrast reflects the capacitive losses caused by cell membranes.

Q. What is the difference between whole body and segmental BIS?
A. There are different approaches to bioelectrical impedance analysis:
  • Segmental-BIS is performed by placing sensor electrodes on body segments, e.g. between ankle and knee to measure calf impedance. It may be helpful for continuous intradialytic monitoring like relative blood volume monitoring.
  • The Fresenius Medical Care approach is a whole body bioimpedance spectroscopy measurement with electrodes placed on the hand and foot only. It determines current fluid status and quantifies overhydration. The method is ideally suited for interdialytic monitoring. Together with the BVM, the BCM – Body Composition Monitor allows continuous fluid status monitoring both during and in between dialysis therapy.

Background Information

Q. Why do we need a body composition model?
A. The body composition model describes the intra and extracellular water content of lean tissue mass (LTM), adipose tissue mass (ATM) and excess fluid (overhydration, OH). LTM, ATM and OH are obtained from measurements of body weight, height and whole body intracellular water (ICW) and extracellular water (ECW) determined by bioimpedance spectroscopy. The body composition model determines whether changes in intra- and extracellular water reflect increase or loss of ATM or LTM.

Q. How are the output parameters validated?
A. The determination of fluid volumes has been validated against "gold standard" reference methods:
  • Extracellular water – bromide dilution
  • Intracellular water – total body potassium (TBK)
  • Total Body Water – deuterium dilution
  • Lean Tissue Mass – Dual Energy X-ray Absorptiometry (DEXA)
  • Adipose Tissue Mass – 4- compartment modelling, DEXA, Air displacement plethysmography and under water weighing
  • Body Cell mass – magnetic resonance tomography, TBK

There is no existing gold standard reference for the determination of OH, but a high agreement has been demonstrated in comparison with expert clinical assessment and the comparison to the ultrafiltration volume withdrawn during a hemodialysis treatment

Q. When I drink 1 litre of water why don’t I see that in the measurement?
A. The BCM – Body Composition Monitor performs whole body measurements and is particularly sensitive to fluid accumulation in the limbs. It detects fluid almost entirely on a tissue level and in the blood vessels. As fluid is the best electrical conductor in the body, a volume of fluid with large cross sectional area has little influence on the measured whole body impedance. The trunk for example contributes as little as 10% to the whole body impedance. Consequently, in PD patients it makes little difference whether the measurements are made before or after drain out as the filling of the peritoneum will have virtually no effect on the calculation of overhydration. However the weight of the patient will be directly affected, thus influencing the calculation of body composition. It is advisable therefore that the weight obtained with an empty peritoneum is entered into the device.

Q. How is OH calculated?
A. The BCM - Body Composition Monitor expresses the body weight in terms of lean tissue mass (LTM – mainly muscle), adipose tissue mass (ATM – mainly fat) and overhydration (OH). Each of these compartments has a specific composition and contains a known quantity of water per mass of tissue. OH is almost 100% extracellular water, whereas the water of LTM and ATM consist of differing proportion of extracellular and intracellular water in addition to solid components. Healthy individuals are considered to be “normally hydrated" and therefore have virtually no overhydration. These individuals may be characterized in terms of ATM and LTM only. However, in pathophysiological conditions the presence of excess fluid that may accumulate has to be taken into consideration. Excess fluid represents an expansion of only the extracellular water, whereas ICW remains unchanged. The excess fluid may reside within adipose tissue or lean tissue raising the hydration of the respective tissue above the “normal” values (e.g. oedema). Alternatively, excess fluid may simply appear as a distinct compartment without altering the hydration of the major tissues (e.g. ascites, pleural effusion). As the extracellular hydration of LTM and ATM is known, the expected “normal” volume of ECW of these tissues can be calculated. The difference between "normal" ECW and measured ECW is the excess fluid, OH.

Q. What does negative OH mean?
A. Negative OH means that the patient is under- or dehydrated. The likelihood of intradialytic symptoms increased in these patients.

Q. What is the basis of the reference ranges?
A. Reference ranges are available for overhydration (OH), lean tissue index (LTI), fat tissue index (FTI) and E/I ratio. The reference ranges are defined by a reference population of 2071 healthy subjects between 2 and 95 years with a BMI between 18 - 35 kg/m2. The reference ranges are defined by the 10th and 90th percentiles of the reference population and are specific to age and gender. These ranges simplify identification of abnormal conditions by comparing a patient’s data to the reference population.


Q. Can I measure regional differences in body composition and fluid status?
A. The BCM - Body Composition Monitor performs whole body measurements between hand and foot and therefore cannot assess regional differences in body composition and fluid status. The device only detects interstitial fluid - a volume of fluid with a large crosssectional area has little influence on the whole body impedance.

Q. Can I measure in the ICU setting?
A. No, currently the BCM - Body Composition Monitor has no approval for ICUs (electrical safety, conformity).

Q. What has to be taken into account for patients with a pacemaker or with stents?
A. There are no limitations for patients with stents or patients with bipolar pacemakers.
Patients exhibiting massive cardiac decompensation who wear a unipolar pacemaker and are entirely dependent on this device, check the sensitivity threshold of the pacemaker before performing the measurement. Measurement must not be performed on patients with a pacemaker whose sensitivity threshold is very low.