What tissue-water asymmetry tells us - and what it doesn’t
When measuring an arm with LymphScanner, a clinician receives two closely related pieces of information:
PWC - Percentage Water Content
The local superficial tissue water content at the measurement site.
PWC ratio
A comparison between the PWC measured at one location and an appropriate reference location.
For unilateral arm assessment, corresponding locations on the at-risk and contralateral arms can be compared. The ratio places the local measurement into the context of the patient's own reference tissue.
But what does a ratio of 1.20 actually mean?
And just as importantly, what does it not mean?
Understanding this distinction is fundamental to interpreting local tissue-water measurements correctly.
Start with the simplest question: what does the ratio measure?
LymphScanner measures local superficial tissue water content at the measurement site.
For an inter-arm comparison:
At-risk or affected-side PWC ÷ corresponding healthy-side PWC = PWC ratio
For example:
A ratio of 1.20 means that the measured local PWC at that site is 20% higher than at the corresponding site on the reference arm.
It does not mean that:
- the whole arm contains 20% more fluid,
- arm swelling has increased by 20%,
- the patient has “20% lymphedema,” or
- the number represents the overall severity of lymphedema.
The distinction is simple but important:
PWC tells us about the superficial tissue water at one site.
The ratio tells us how different that site is from its reference.
Why use a ratio at all?
Absolute superficial tissue water naturally varies between individuals and anatomical locations.
Consider three examples:
The first two examples have exactly the same at-risk-side PWC of 36%, but the relationship between the two arms is very different.
The third example is perhaps even more instructive. The PWC is only 30%, yet it is 25% higher than the corresponding location on the other arm.
This illustrates why asking only:
“Is a PWC of 36% high?”
is often less useful than asking:
“How different is the superficial tissue water here compared with the corresponding tissue on the other arm?”
A ratio turns an isolated local measurement into a within-person comparison. This helps account for some of the natural biological variation between people.
So what does 1.20 mean?
An inter-arm ratio around 1.20 has substantial historical support in TDC research as a reference threshold for unilateral upper-extremity lymphedema assessment.
A simple way to understand the ratio itself is:
1.00
Equal PWC at the two locations
↓
1.10
10% higher PWC at the measured location
↓
1.20
20% higher PWC
↓
Increasing local tissue-water asymmetry
The important concept is therefore not simply the number 1.20.
It is asymmetry.
A ratio around 1.20 represents a relatively large difference between corresponding arm locations compared with the side-to-side variation generally observed in healthy arms.
But to understand why 1.20 became important, it helps to understand where the reference came from.
Where did the 1.20 reference come from?
The 1.20 value was not chosen simply because a 20% difference sounded clinically important.
It emerged from research examining the natural variation in Tissue Dielectric Constant (TDC) between arms.
In a 2014 study by Mayrovitz and colleagues, inter-arm TDC ratios were evaluated in women with and without breast cancer treatment-related lymphedema.
One statistical approach was to establish a reference boundary at three standard deviations above the mean inter-arm ratio in the non-lymphedematous reference group.
This produced a calculated value of 1.196, which was rounded to 1.20.
All women in the non-lymphedematous sample were below 1.20, while 78 of 80 women with lymphedema were above it.
This gives the 1.20 reference an important meaning:
It represents a degree of inter-arm tissue-water asymmetry that is uncommon between healthy arms.
That is different from saying that 1.20 is a biological boundary between “healthy” and “lymphedema.”
Research helps us understand the probability continuum
More recent research provides another perspective on normal inter-arm variation.
A 2026 study evaluated inter-arm TDC differences in 403 healthy women. Instead of defining one arm as the affected or at-risk arm, the researchers calculated a direction-independent ratio by dividing the arm with the higher TDC value by the arm with the lower value. This ratio was called β (beta).
The average β ratio was approximately 1.052.
As the ratio increased, the observed difference became progressively less typical of the healthy reference population:
Only approximately 0.74% of the healthy women had a β ratio greater than 1.20.
This provides a useful way of thinking about inter-arm asymmetry:
As the difference between the arms increases, that degree of asymmetry becomes progressively less common in healthy individuals.
However, the newer β ratio and the traditional clinical inter-arm ratio are not exactly the same calculation.
The β ratio always compares the arm with the higher value with the arm with the lower value. In clinical unilateral arm assessment, the comparison is between an at-risk or affected arm and the contralateral reference arm.
For this reason, values such as ~1.17 from the healthy-reference study should not be interpreted as new clinical thresholds replacing 1.20. Instead, the study helps us better understand the continuum of normal inter-arm variation.
1.20 is not a biological switch
This is one of the most important concepts when interpreting the ratio.
Human biology does not suddenly change when a ratio moves from:
1.19 → 1.20 → 1.21
There is no physiological switch at exactly 1.20.
Instead, think of tissue-water asymmetry as a continuum.
As the ratio moves farther from the relationship normally observed between healthy corresponding arm locations, the asymmetry becomes increasingly unusual.
The 1.20 reference provides a useful point on that continuum.
It helps answer:
“Is there a substantial local difference in superficial tissue water between these corresponding arm locations?”
It does not independently answer:
“Does this patient have lymphedema?”
LymphScanner measurements should therefore support clinical judgment rather than replace it.
A ratio belongs to a location - not to the whole arm
This is particularly important when working with localized measurements.
Imagine the following measurements:
It would be misleading to summarize this simply as:
“The patient has a ratio of 1.20.”
A more informative interpretation is:
“There is a localized tissue-water asymmetry at the measured forearm location that is not present to the same degree at the other measured locations.”
Why?
Because LymphScanner is a local measurement.
The forearm measurement describes the forearm location. The upper-arm measurement describes the upper-arm location. Neither automatically describes the whole limb.
Research has also demonstrated differences between upper-arm and forearm TDC ratios in patients with breast cancer-related lymphedema, supporting the concept that superficial tissue-water abnormalities are not necessarily distributed uniformly throughout the limb.
This gives us a useful rule:
A local measurement answers a local question.
Compare like with like
For unilateral arm assessment, ratios should be based on anatomically corresponding locations whenever possible:
Forearm ↔ forearm (similar specific anatomical spots)
Upper arm ↔ upper arm (similar specific anatomical spots)
Hand ↔ hand (similar specific anatomical spots)
The purpose of the comparison is not to assume that the two arms have identical lymphatic drainage pathways.
The purpose is to compare similar tissue locations within the same individual.
This is also why consistent anatomical positioning becomes especially important when measurements are repeated over time.
Local and whole-limb measurements may tell different stories
Circumference, arm volume, palpation, symptoms and local PWC measurements provide different perspectives on the patient.
A localized tissue-water abnormality can be substantial without yet producing a large change in total arm volume.
Conversely, limb size can change for reasons that do not necessarily produce an equivalent change in local superficial tissue water.
The different assessment methods should therefore generally be viewed as complementary rather than competing.
Research in early breast cancer-related lymphedema has shown that local TDC findings and whole-arm measures do not always identify the same patients or the same pattern of change.
The clinical lesson is simple:
A local measurement and a whole-limb measurement do not have to tell exactly the same story because they are measuring different aspects of the patient.
For example, a patient can have only a small difference in total arm volume while showing substantial local superficial tissue-water asymmetry at one part of the arm.
This is one reason why localized measurement can add information to the broader clinical assessment.
A ratio below 1.20 at one site does not mean “no lymphedema”
This follows directly from the local nature of the measurement.
Suppose the forearm ratio is 1.05.
What can we conclude?
We can say that there is little tissue-water asymmetry at those measured forearm locations.
We cannot automatically conclude that no abnormality exists elsewhere in the arm.
Lymphedema can be regional and heterogeneous. One part of the arm may show increased superficial tissue water while another remains relatively symmetrical.
This is why standardized measurement sites are useful, but why mapping clinically interesting areas can provide additional information when symptoms, palpation or other findings suggest a localized abnormality.
A low ratio at one site should therefore not be generalized to the entire limb.
PWC and the ratio answer different questions
Consider two patients:
Both have exactly the same ratio.
But their absolute PWC values are clearly different.
Therefore:
PWC asks:
What is the local superficial tissue water here?
The ratio asks:
How different is this location from its reference?
Both pieces of information can matter.
The ratio should not make the underlying PWC values disappear.
Baseline and trend add another dimension
Now consider repeated measurements at the same corresponding anatomical sites:
1.04 → 1.09 → 1.15 → 1.21
If we look only at the last visit, the interpretation becomes:
“The ratio crossed 1.20.”
But longitudinally, something more informative has happened:
Local tissue-water asymmetry has progressively increased.
That does not by itself establish that lymphedema has progressed. Measurement quality, anatomical location, symptoms and the wider clinical picture still need to be considered.
But it demonstrates why baseline and repeated measurements can provide information that a single threshold cannot.
A useful distinction is:
The threshold provides a reference for today's asymmetry.
The baseline provides a reference for how the patient is changing over time.
The same principle works in the other direction.
If a ratio changes:
1.65 → 1.51 → 1.34
the appropriate interpretation is:
Local superficial tissue-water asymmetry between those measured locations has decreased.
It would not be correct to say:
“The patient's lymphedema decreased by 31%.”
The ratio describes a local relationship, not the total amount or severity of lymphedema.
What if both arms change?
Ratios also have limitations.
Imagine:
The ratio has not changed.
But the absolute PWC measurements have.
This illustrates another important principle:
Always retain the underlying PWC values.
A ratio can remain stable while absolute PWC changes.
Side-to-side ratios also require particular caution when both sides may be affected, because one side may no longer provide an appropriate healthy reference.
Does the 1.20 reference apply everywhere?
No.
The 1.20 reference is specifically associated with inter-arm assessment and should not automatically be transferred to every anatomical region.
The appropriate comparison depends on the anatomy, expected physiological variation, available reference tissue and the evidence supporting that particular measurement approach.
Examples from published research and Delfin measurement protocols include:
These values should not be interpreted as interchangeable universal diagnostic thresholds. The evidence and reference methods differ between anatomical regions.
The lower-extremity approach is particularly interesting because it illustrates why the choice of reference matters.
Rather than relying on the opposite leg as the reference, a volar forearm measurement can be used. This is useful because lower-extremity lymphedema can be bilateral, making the opposite leg potentially unsuitable as a healthy reference.
This leads to a broader principle:
PWC is the local tissue-water measurement. The ratio is a way of placing that measurement into context using an appropriate reference.
Different anatomical regions may therefore require different reference strategies.
How should clinicians use the 1.20 reference?
Instead of asking only:
“Is the ratio above or below 1.20?”
consider the measurement in a broader clinical context:
- Where was the measurement taken?
- Were anatomically corresponding sites compared?
- What are the underlying PWC values?
- How large is the tissue-water asymmetry?
- Is the finding localized or present at multiple sites?
- Has the PWC or ratio changed from baseline or previous measurements?
- Does the finding correspond with the patient's symptoms, palpation, circumference or volume findings and wider clinical picture?
This changes the role of 1.20.
Instead of being the end of the assessment, it becomes one important reference point within the assessment.
A simple clinical model
Four questions can help organize LymphScanner information:
PWC
What is the superficial tissue water here?
RATIO
How different is here from an appropriate reference?
MAPPING
Where is the asymmetry located?
TREND
How is that relationship changing over time?
Or, even more simply:
Measure → Compare → Map → Follow
This way of thinking is particularly useful in prospective surveillance. Rather than waiting for one measurement to cross a threshold, clinicians can establish a baseline, measure consistently, recognize changing patterns and bring that information into the broader clinical assessment.
Selected references
Mayrovitz HN, Weingrad DN, Davey S. Tissue dielectric constant (TDC) measurements as a means of characterizing localized tissue water in arms of women with and without breast cancer treatment related lymphedema. Lymphology. 2014;47.
Mayrovitz HN. Interarm Tissue Dielectric Constant Ratios: Reference Values Derived From Healthy Women. Lymphology. 2026;59(1):20–26.
Lahtinen T, Seppälä J, Viren T, Johansson K. Experimental and analytical comparisons of tissue dielectric constant (TDC) and bioimpedance spectroscopy (BIS) in assessment of early arm lymphedema in breast cancer patients. Lymphatic Research and Biology. 2015.
Mayrovitz HN. Assessing Lower Extremity Lymphedema Using Upper and Lower Extremity Tissue Dielectric Constant Ratios: Method and Normal Reference Values. Lymphatic Research and Biology. 2019;17(4).
Mayrovitz HN, et al. Medical Applications of Tissue Dielectric Constant Measurements of Localized Tissue Water. Cureus. 2023.
Delfin Technologies. LymphScanner Measurement Protocol — Arm.
Delfin Technologies. LymphScanner Measurement Protocol — Breast.
Delfin Technologies. LymphScanner Measurement Protocol — Leg.
Indication of Use
FDA clearance 510(k) in USA: LymphScanner is a device utilizing inter-aim ratios of tissue dielectric constant (TDC) that supports local assessment of tissue water differences between affected and contralateral non-affected arm tissues to aid in forming a clinical judgment of unilateral lymphedema in adult women. (K220557)
EU: LymphScanner is used to aid in forming a clinical judgement of edema at any soft tissue site of the body
Key takeaway
The 1.20 inter-arm ratio is a well-established historical reference for interpreting local superficial tissue-water asymmetry in unilateral upper-extremity assessment.
Its importance comes from the observation that large differences between corresponding healthy arm locations are uncommon. Historical research established approximately 1.20 as a practical reference, while newer healthy-reference data further demonstrate that asymmetry approaching this level lies toward the extreme end of normal inter-arm variation.
But 1.20 is not a biological switch.
A ratio of 1.19 should not automatically be considered normal while 1.20 is automatically considered lymphedema. The ratio should not be interpreted as a stand-alone diagnosis or as a measure of the total severity of lymphedema.
A ratio also belongs to the specific anatomical locations being compared.
The clinical objective is therefore not simply to search for a number above 1.20.
It is to understand:
What is the local superficial tissue water?
How different is it from an appropriate reference?
Where is the asymmetry located?
How is that pattern changing over time?
Together with symptoms, clinical examination and other measurements, this can provide a more complete picture of what is happening in the patient's tissue
