Are sub-Q fluids enough? Ringer's doesn't hydrate the cells (sodium and free water)
You give subcutaneous fluids every single day, yet the blood test shows sodium going up, and the scale shows your cat heavier than usual. After all that fluid, why does the lab say "short of water"?
The answer is where the water goes. The Ringer's solutions given under the skin (Hartmann's, lactated Ringer's, acetate Ringer's) are good at filling the blood vessels and the tissue around them, but they barely reach the inside of the cells. What fills the cells is the water your cat drinks, the water in food, and dilute, low-sodium fluids.
- Of 100 ml of Ringer's, only about 10 ml reaches the inside of the cells. The rest stays outside them, in the blood and between the tissues. Of 100 ml of water your cat drinks, two thirds goes into the cells.
- 200 ml of Hartmann's carries about 600 mg of sodium, 3.7 times what a 4 kg cat eats in a day of dry renal food (about 160 mg).
- In a cat that drinks well, the jobs split neatly: sub-Q fluids for the vessels, drinking for the cells. In a cat that has stopped drinking, simply giving more isotonic fluid can pile up sodium while the cells dry out.
- The 2026 iCatCare CKD guidelines: ideally a hypotonic fluid (half-strength lactated Ringer's, 0.45% saline and others) is used to reduce the sodium load, but in practice cats tolerate other fluids without problems.
- If sodium is climbing, the order is ① more drinking → ② dilute the fluid (start at 2:1 with water for injection) → ③ adjust the volume itself. Giving more fluid may not be the answer.
| Who | Decides |
|---|---|
| You, at home | Recording daily water intake (ml) and weight, bowl placement and height, adding water to food |
| Together | Fluid type, whether to dilute with water for injection and at what ratio, daily fluid volume |
| The clinic | Correcting sodium above 170 or neurological signs (IV), how much potassium to add, judging the heart and lungs |
1. The body keeps its water in two rooms
About 60% of a cat's body weight is water: roughly 2.4 L in a 4 kg cat. That water lives in two compartments.
- Outside the cells: about one third (0.8 L). The blood in the vessels and the fluid between cells. Blood pressure and blood flow to the kidneys (perfusion) depend on this room.
- Inside the cells: about two thirds (1.6 L). The water that brain, muscle and organ cells work in.
The wall between the two rooms (the cell membrane) lets water through but holds sodium back. So water is pulled toward whichever side has more sodium. A fluid with roughly the body's sodium concentration (isotonic) has no reason to pull water anywhere and stays outside the cells. Plain water with no sodium spreads evenly through both rooms, so two thirds of it ends up inside the cells.
| Give 100 ml of | Stays outside the cells | Goes inside the cells |
|---|---|---|
| 0.9% saline | about 100 ml | 0 ml |
| Hartmann's / lactated Ringer's | about 90 ml | about 9 ml |
| Hartmann's + water for injection 1:1 (half strength) | about 63 ml | about 37 ml |
| Drinking water / water in food | about 33 ml | about 67 ml |
Approximations, assuming total body water splits 1/3 outside and 2/3 inside the cells, and that only the sodium- and potassium-free share of a fluid (its free water) spreads into both rooms.
That is why sub-Q fluids are strong at protecting the vessels and kidneys, and drinking is strong at hydrating the cells. Neither fully replaces the other. It is also why "if she won't drink, sub-Q fluids will cover it" is only half true.
2. The sodium in 200 ml of Ringer's
- Sodium in Hartmann's: 130 mmol per litre
- 200 ml = 130 × 0.2 = 26 mmol
- 1 mmol of sodium = 23 mg, so 26 × 23 = about 600 mg
- For comparison: the median sodium of 27 dry renal diets in our database is 807 mg per 1000 kcal. A 4 kg cat eating 200 kcal a day gets about 160 mg
- So the fluids deliver about 3.7 times the sodium in the food
More sodium is not automatically a problem. In a study that fed 20 healthy senior cats a diet with 3.26 g of sodium per 1000 kcal (three times the control diet) for up to five years, kidney function, blood pressure and the heart did not differ (the PEANUT study, Reynolds 2024, funded by Royal Canin). But those cats got rid of the sodium with healthy kidneys, while drinking freely. High-salt diets are in fact used precisely to make cats drink more and dilute their urine.
The 200 ml of Hartmann's above works out to about 3.0 g per 1000 kcal. Add the sodium in the food, and the cat takes in more than that study's high-salt diet, through failing kidneys, while hardly drinking.
3. Why sodium rises when you are giving fluids
Incoming sodium has to leave in the urine. But kidneys with CKD cannot concentrate urine. If the urine's sodium concentration is lower than that of Ringer's (130), then getting rid of the sodium from the fluid costs more water than the fluid brought in. That shortfall has to be made up by drinking, and a nauseous cat that drinks less does not make it up.
Two things then happen at the same time.
- The outside room overfills: the leftover sodium holds water, so weight goes above usual, and cats with low albumin swell easily in the legs and belly.
- The inside room dries out: as blood sodium rises, water is drawn out of the cells. That is hypernatremia.
- Weight down + sodium up: short of water overall (ordinary dehydration). More fluid and more water both help.
- Weight up + sodium up: "too much volume, yet dry cells". More fluid only swells the outside room. Drinking plus diluted fluid come first.
- Weight up + normal sodium + faster breathing: suspect too much fluid. See the pulmonary oedema section of the sub-Q fluids article.
How short is the cat? The free water deficit
- Water deficit (L) = 0.6 × body weight × (current sodium ÷ target sodium − 1)
- = 0.6 × 4 × (160 ÷ 152 − 1) = 2.4 × 0.053 = about 0.13 L = 130 ml
- Just two days of drinking 60–70 ml less than usual adds up to this much.
Around 160, sodium causes little more than poorer appetite and low energy, which is hard to tell apart from CKD itself. Above 165–170 the dullness becomes obvious, and above 170 neurological signs such as wobbling and seizures can appear. In other words, part of "she's nauseous and eating less these days" may be sodium.
4. First fix: more drinking
This is the most natural way to fill the cells, and it costs nothing and risks nothing. Healthy cats take in 23–51 ml of water per kg of body weight a day from food and drinking combined (a review of 32 studies, Kosmal 2026), so 90–200 ml for a 4 kg cat. Cats with CKD lose more in urine, so aim for the upper end.
The 2026 iCatCare guidelines suggest:
- Water in several places: a fountain alongside wide, shallow ceramic bowls. Always put one near where she sleeps. A sick cat will not walk far for a drink.
- Raise the bowl: cats with arthritis find it hard to lower their head and drink less. Lifting the bowl a few centimetres on a stand makes a difference.
- Wet food: cans are 75–80% water. For the same calories, that is about 150 ml more water than dry food.
- Water in the food, as long as calorie intake does not drop. Add a few ml to liquid diets such as Recovery as well.
- Nutrient-enriched water: shown to increase voluntary drinking in dehydrated cats.
If nausea is why she drinks less, treating the nausea first is also a way of increasing water. In the app's meal log you can record water as "drank on her own / mixed into food / syringe-fed", so you can see which one actually dropped.
5. Second fix: dilute the fluid
For a cat who cannot be persuaded to drink more, you can mix sodium-free water into the sub-Q fluid itself to increase the share that reaches the cells. The 2026 iCatCare guidelines say this about home sub-Q fluids (75–100 ml every 1–3 days):
(Taylor et al., J Feline Med Surg 2026)
| Fluid (100 ml) | Sodium | Potassium | Buffer | Free water |
|---|---|---|---|---|
| 0.9% saline | 354 mg | none | none | 0 ml |
| Acetate Ringer's Plasma-Lyte A | 322 mg | 5 mEq/L | acetate + gluconate | about 6 ml |
| Hartmann's / lactated Ringer's | 299 mg | 4 mEq/L | lactate 28 | about 13 ml |
| Ringer's 2 : water for injection 1 | about 200 mg | about 2.7 | two thirds | about 42 ml |
| 0.45% saline | 177 mg | none | none | 50 ml |
| Ringer's 1 : water for injection 1 (half strength) | about 150 mg | about 2 | half | about 56 ml |
Free water = the share of 100 ml left after taking out sodium and potassium at body concentration (approximation using 154). Two thirds of it goes into the cells.
Why we look at "Ringer's + water for injection" before 0.45% saline
0.45% saline is low in sodium but has no buffer and no potassium. Acidosis and low potassium are common in cats with CKD (especially at later stages), so swapping Ringer's out entirely for 0.45% saline can make acidosis worse again. Mixing water for injection into Ringer's lowers the sodium while keeping part of the buffer and potassium. For a cat who has had acidosis, acetate Ringer's makes the better base.
- Start at 2:1 (one third water), recheck sodium after 1–2 weeks and go to 1:1 if needed. Never let water be more than half (1:1 is the guidelines' "half strength" and the limit).
- If you give fluids by syringe, mixing in the syringe, dose by dose is the cleanest way. Example: 40 ml Ringer's + 20 ml water for injection in a 60 ml syringe.
- Use only sterile water for injection. Not sterile water for irrigation, distilled water, purified water or bottled water.
- Use the mix immediately. Do not prepare it ahead. Once you open an ampoule or vial of water for injection, use it all for that session.
- Clean the needle and port as you normally would for sub-Q fluids. Fluids containing glucose do not go under the skin.
6. Bring sodium down slowly
When sodium has been high for more than a day or two, brain cells protect themselves from shrinking by concentrating their own contents. If sodium then drops suddenly, water rushes into those brain cells and they swell. The standard limit for correction is 0.5 per hour and 10 mmol/L per day.
Sub-Q fluids are absorbed slowly over several hours and in small volumes, so they rarely exceed that pace. Even switching a 4 kg cat (about 2.4 L of body water) to 200 ml a day at half strength lowers sodium by at most about 7 on paper. The danger lies in giving dilute fluid fast, intravenously, and that is the clinic's territory.
7. Where opinions differ
Is a hypotonic fluid really necessary?
The guidelines' own sentence holds both sides: "ideally hypotonic", yet "in practice other fluids are fine". No clinical trial has compared sub-Q fluid types head to head. In a survey of 399 owners giving sub-Q fluids (Cooley 2018), the most common volume was 100 ml, and at that amount every day or two, sodium problems are uncommon.
Our view: if she drinks well and sodium is normal, keep the Ringer's you are using. Reasons to change appear when water intake has dropped, fluids exceed 150 ml a day, or sodium is climbing toward the top of the range. At that point, look at diluting before giving more.
How to check in your own cat: compare sodium, potassium and weight before the change and 1–2 weeks after, using the same lab. If sodium comes down and potassium holds, you are heading the right way.
Sodium and blood pressure
In people, the evidence that salt raises blood pressure is solid. Cats are different. Feeding healthy senior cats a high-salt diet for two years (Reynolds 2013, 16 cats completed) and five years (Reynolds 2024, 20 cats) produced no change in blood pressure. Both studies used healthy cats, though, and data in cats with CKD or existing hypertension are lacking.
Our view: we do not claim that fluid sodium raises blood pressure. If your cat is on blood pressure medication, measuring blood pressure at home once more after increasing or changing the fluids is enough.
8. What to bring to the clinic
Bring your records before your questions. These four things on one page make the conversation short and precise:
- Sodium and potassium from the last 2–3 tests (with the lab name and reference range)
- Daily water intake (ml) over 1–2 weeks, separating what she drank herself from what was mixed into food
- Fluid type, volume per session and frequency
- Weight over the same period
With those in front of you, the questions come naturally:
- "Her water intake has halved and sodium went from 152 to 160. Could we mix part of the fluid with water for injection, starting at 2:1?"
- "She had acidosis. When we dilute, would acetate Ringer's be the better base?"
- "If potassium drops after diluting, how much potassium should we add to the fluid?"
9. Read next
- Subcutaneous fluids at home, safely: volume, speed and pulmonary oedema: baseline volumes and warning signs
- Metabolic acidosis: why acetate fluids are used
- Potassium: a problem whether high or low
- The bowl is empty, but who drank it? Recording water intake
- Fluid therapy guide (2024 AAHA): the principles behind clinic fluids