Published: April 21, 2024 | 9 mins read

Uric Acid Kidney Stones: Subtypes and Causes


Uric acid (UA) kidney stones are the second most common kidney stone type, comprising about 15% of the cases worldwide. Unfortunately, the number of UA stone-formers is on the rise. As always, the leading cause of this trouble is DIET.

However, if you think uric acid is the main culprit, you are missing the mark. Even people with normal uric acid levels can form this type if you have the one thing that increases your risk.

What’s that one thing?

Let’s find out in the following section.

CAUSES OF URIC ACID STONES

 Three factors contribute to uric acid stone formation: low urine volumeurinary uric acid, and acidic urine.

Typically, uric acid can be dissolved in the urine. However, its solubility decreases when the urine becomes acidic (< 5.5 pH). That means uric acid crystallizes under an acidic urine environment. Interestingly, it dissolves six times higher at a urine pH of 6.5 compared to pH 5.3.

Four studies showed patients with uric acid stones have a urine pH below 6. A low urine pH caused uric acid stones even when the total uric acid excretion was normal. This proves that the primary culprit for this stone type is acidic urineUric acid alone won’t be a problem if you maintain a neutral urine pH (6.0-7.0).

Curious enough, what could be driving an acidic urine?

Often than not, metabolic syndrome and diabetes, which point to poor diet, cause this extra acidity.

Metabolic Syndrome

Metabolic syndrome is a cluster of risk factors, including the following:

Metabolic Syndrome Traits

A study involving 5000 kidney stone-formers found that obesity and high blood sugar levels (fasting glucose over 100mg) result in acidic urine. This is due to decreased ammonium production in the kidney. Ammonium is an alkaline ion, so reduced levels can lead to acidic urine.

Meanwhile, hypertension and dyslipidemia (high triglycerides and low HDL) don’t necessarily lower urine pH. However, they can affect the blood vessels throughout the body, including the kidneys. This can lead to inflammation and constriction, which reduces urine output.

Also, inflammation can favor the formation of plaques (Randall’s Plaque) where calcium oxalate (CaOx) stones thrive. In that sense, hypertension and dyslipidemia are risk factors not only for UA stones but also for this other type.

Now that we understood the impact of metabolic syndrome, let’s talk about diabetes.

Diabetes

There are two types of diabetes. Type 1 happens when the immune system mistakenly destroys the insulin-producing cells in the pancreas. The body can’t produce enough of this hormone, which is responsible for knocking on the cells to let glucose (sugar) in for energy. On the other hand, Type 2 is characterized by insulin resistance. The pancreas produces enough insulin, but the cells can’t recognize it, so glucose will stay in the blood.

Both Type 1 and 2 diabetes cause acidic urine because they alter how sugar is regulated in the body. Most of the time, they result in more sugar in the blood, which, as explained earlier, causes less ammonium production in the kidney. This makes the urine more acidic.

Lastly, what role does low urine volume play in this drama?

Low Urine Volume

As a general rule, if you don’t urinate often, stone-forming materials sit longer in your kidneys. It’s equal to giving them all the chances in the world to bind together, form stones, and trouble your life. So, you have to maintain proper hydration to prevent whatever stone type. When we say proper hydration, we mean drinking about 96 oz. of water daily. Drinking this much is a key point in your prevention plan

THE SUBTYPES OF URIC ACID KIDNEY STONES 

Just like other stone types, uric acid kidney stone is also an umbrella term. There are different subtypes of it depending on how they distinctively form. Below is a comprehensive list.

TYPE IIIa STONES

This type usually has the following characteristics:

  • Has an even and smooth surface
  • Typically orange, sometimes cream, ochre, or yellowish in color

Low urine pH, stasis (urine retention), prostate enlargementmetabolic syndrome, or defect in ammonium production are the causes of this type.

Type IIIa Stones

TYPE IIIb STONES

These stones are characterized as follows:

  • Has an unevenly embossed surface
  • Rough and porous in texture
  • Beige to brown-orange in color

The leading causes of this type are insulin resistancemetabolic syndrometype 2 diabetes, defect in ammonium production, and low urine pH.

Type IIIb Stones

TYPE IIIc STONES

Type IIIc stones have these attributes:

  • May appear slightly rough and locally porous on the surface
  • Usually cream to grayish in color

These are caused by high urinary uric acid and urinary tract infection by urea-splitting bacteria. You can have this type even if your urine pH is neutral or alkaline.

Type IIIc Stones

TYPE IIId STONES

These stones have the following:

  • Uneven, rough and porous surface
  • Grayish to dark brown color

These stones form due to chronic diarrhea, electrolyte/alkali loss, high urate (uric acid salt) concentration in urine, low phosphate intake, or laxative abuse (medications for loosening stool).

Aside from these four, there are also mixed uric acid and calcium-oxalate stones. These mixed stones form when you have risk factors for both types. They are typically observed in individuals who are obese. They are especially severe and recurrent. Since we only covered UA stones in this blog, we suggest that you also dive into our Calcium Oxalate Stone Blog to know the risk factors for this type.

Type IIId Stones

TYPE Ia/IIIb/IVa + Ib/IVa STONE

This type is characterized as follows:

  • Smooth surface
  • Has a prominent umbilication
  • Has yellowish to brownish color

Underneath this yellowish surface, you will notice a whitish internal structure with brownish lines (somehow like rings). There are also patches of black.

These stones form due to intermittent (on and off) or moderately high oxalate concentration and past hypercalciuria (excess urinary calcium). These are in addition to UA stone risk factors such as low urine outputmetabolic syndromeinsulin resistance, and type 2 diabetes.

Mixed UA and CaOx Type 1

TYPE Ia/IIIb STONE

These stones have embossed yellowish or brownish layers that are unevenly distributed against the white and brown surface.

Inside, you will notice some layers that are dark brown to black in color. These irregularities in texture and hue are due to moderately high oxalate concentrationlow urine outputacidic urinemetabolic syndrome, and type 2 diabetes.

Mixed UA and CaOx Type 2

TYPE IIa + IIIb/Ib STONE

These stones have these attributes:

  • Spiky but less prickly surface
  • Light to yellowish color

The internal structure shows concentric layers of brown and black. This signals a transient rise in uric acid and/or low urine pH, plus recent increases in urinary calcium and oxalates.

Mixed UA and CaOx Type 3

TYPE IIIa/IIIb +Ia STONE

This type has the following:

  • Slightly rough surface
  • Has white color with patches of orange or orange-brown

These stones have a brown internal structure and a prominent black core. These stones form primarily due to metabolic syndrometype 2 diabetes, or insulin resistance.

Mixed UA and CaOx Type 4

Another sub-category of UA stones are ammonium urate stones. Although they account for less than 0/4% of all kidney stones, interestingly, they are frequent in regions such as Africa and Southeast Asia. They have two different forms:

TYPE Ia/IIIb/IIIa/IIIc STONE

This subtype is characterized as follows:

  • Having a rather smooth but blistery-like surface
  • Light yellow to light brown in color

This type has three layers: the yellowish surface, the black middle layer, and the whitish core. This distinctive appearance is due to increased uric acid levels, a moderate oxalate rise, and past urinary tract infections or type 2 diabetes.

Ammonium Urate Type 1

TYPE IIa/IVa1 + IIb + IIId STONE

These stones are characterized by:

  • Rough surface
  • Light yellow color

Internally, these stones have a brownish-yellow color with concentric layers, usually with lines of black. This results from episodes of diarrhea, high urinary uric acidhigh oxalate concentrationlow diuresis, and a recent rise in urinary calcium of dietary origin.

Ammonium Urate Type 2

HOW TO MANAGE URIC ACID KIDNEY STONES

Treating UA stones isn’t as complicated as most people think. Honestly, for most individuals who form this type, surgery is almost unnecessary. This must be surprising on your part, but that’s the truth. Uric acid stones are weak-density stones. Meaning they can easily break apart while still inside the urinary tract. Some potent natural supplements, like CLEANSE, contain stone-breaking properties to do the job. Taking CLEANSE will not only help you break your UA stone but also to pass them smoothly. That’s because it has pain relief, anti-inflammatory, diuretic and stone-breaking ingredients.

On the prevention side, you can check out our Coaching Program. In this program, we will help you formulate a personalized strategy on how to combat uric acid stones effectively. See you there!

Uric Acid Kidney Stone Subtypes
Mixed Uric Acid and Calcium Oxalate Subtypes
Ammonium Urate Stone Subtypes

References

  1. Comprehensive morpho-constitutional analysis of urinary stones improves etiological diagnosis and therapeutic strategy of nephrolithiasis
  2. The influence of metabolic syndrome and its components on the development of nephrolithiasis
  3. Diabetes mellitus and the risk of nephrolithiasis
  4. Uric Acid Stones and Hyperuricosuria

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