Specific Gravity: The Hydration Pad That Changes Every Other Reading

Most people skip the specific gravity pad. It reads 1.010 or 1.020 and the number means nothing to them. It is worth learning, because it is the one result that tells you whether the rest of the strip can be believed.

What it measures

Specific gravity is the density of urine relative to pure water. Water is 1.000. Urine carries dissolved salts, urea, and other solutes, so it is heavier. The more concentrated the urine, the higher the number.

The normal range on a random sample is about 1.005 to 1.030. First morning urine usually sits at 1.020 or above, because the kidneys have been concentrating overnight. Urine after a liter of water can drop to 1.005 or lower.

The pad works by a chemical reaction sensitive to ion concentration. It changes from blue green through green to yellow as specific gravity rises. Most strips read it in steps of 0.005.

What it tells you about hydration

Low, around 1.005 or below, means dilute urine. Either you have been drinking a lot, or the kidneys are not concentrating properly.

High, around 1.025 or above, means concentrated urine. Either you have not been drinking much, or something is pushing the kidneys to hold onto water.

For a healthy person the first explanation is almost always the right one, and the pad is a reasonable hydration gauge. Urine color gives the same information for free, but the pad gives a number. See what urine color tells you.

What it tells you about the other pads

Every other pad measures a concentration. A fixed amount of protein or glucose in dilute urine reads lower than the same amount in concentrated urine. Specific gravity is how you correct for that.

A trace protein result at 1.030 is likely just concentration. The same trace at 1.005 means a real amount of protein is present, because it is showing up despite dilution.

A negative nitrite at 1.005 with symptoms is less reassuring than a negative at 1.025, because dilute urine can push a real positive below the detection threshold.

A glucose result at 1.030 first thing in the morning is worth a repeat later in the day to see whether it persists at lower concentration.

This is the reason clinicians look at specific gravity before interpreting anything else on a urinalysis, and the reason first morning urine is the standard sample. Concentration makes the other pads more sensitive.

When the number is abnormal on its own

Persistently low, under 1.005, with normal fluid intake. The kidneys are failing to concentrate. Causes include diabetes insipidus, a condition of the pituitary or kidney unrelated to blood sugar, certain kidney diseases, and some medications including lithium and diuretics. Persistent low specific gravity with unusual thirst and large urine volumes is a reason to see a clinician.

Persistently high, over 1.030. Usually dehydration. If it persists with normal fluid intake, it can point to glucose or protein in urine, which raise density, or to a condition causing water retention. Check the other pads.

Fixed at 1.010 regardless of intake. Called isosthenuria. The kidneys are neither concentrating nor diluting, which means the tubules are not working. This is a finding of advanced kidney disease and would be accompanied by other abnormal labs.

Things that skew it

• Glucose and protein raise it. A strongly positive glucose pad can push specific gravity up by 0.010 on its own.
• Contrast dye from a recent CT scan raises it dramatically for a day.
• Very alkaline urine, above pH 7.5, reads about 0.005 low on the pad. Some manufacturers say to add 0.005 to correct.
• Cold urine reads slightly high. Test at room temperature.

How to use it

Read it first. If it is under 1.010, be cautious about any negative result and repeat on a concentrated morning sample. If it is over 1.025, be cautious about any trace result and repeat on a normally hydrated sample. If it is in between, read the rest of the strip at face value.

That one habit fixes a large share of the false alarms and false reassurances that home urine testing produces. For the full pad list, see what a 10 parameter dipstick measures.