Urinalysis Interpretation Guide

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Specimen Considerations

• First-morning urine improves detection of protein, blood, and casts and are necessary for diagnostics, particularly proteinuria. However, if overly concentrated it may cause false positive proteinuria (since its measured in mg/dL) and blood (some cells lyse due to dehydration).

  • Ordering with urine creatinine alleviates some of the problems with proteinuria in a concentrated sample. ALWAYS order a urine creatinine if checking for proteinuria.

  • Similarly, a very dilute sample can overestimate proteinuria (e.g. a UP/C of 2mg/mg but urine creatinine is 17.8mg is unlikely to be significant- another reason to obtain a first morning sample. I prefer a urine creatinine of at least 40mg/dL.

• Refrigeration slows bacterial growth but promotes crystal precipitation, so urine may appear cloudy.

• Dilute or alkaline urine promotes RBC/WBC lysis and cast dissolution=false negatives and a missed diagnosis.

• Exercise can cause both hematuria and proteinuria

 

*Commonly encountered items  are in bold*​

 

Color:

Red/pink/brown:  Gross hematuria, hemoglobinuria, myoglobinuria, jaundice, phenytoin, nitrofurantoin (brown), Senna (yellow/brown red), Beetroot (red
White: Chyluria
Pink: uric acid crystalluria
Red to black upon standing: Porphyrinuria, alkaptonuria

Yellow/orange: Rifampin

Darkening upon standing Flagyll, 

 

Specific Gravity:

• Measurement of particle/solute in the urine and compares urine to water, the closer to 1.0, the more watery. 1.010 is called isosthenuric- identical SG as plasma. This may indicate kidney injury as it demonstrates an inability to concentrate or dilute. However, many times these UAs are being collected in setting of receiving fluids. If <1.010= hyposthenuric, indicating a concentrating defect (e.g. seen in patients with sickle cell anemia/disease and possibly trait, who can also be 1010).

 

pH:​

• ​Relevant for risk of stone formation (in combination with risk factors), RTA (see "RTA") and in certain clinical scenarios (e.g. clearance of methotrexate).

 

Dipstick Chemistry & Mechanisms

• Specific gravity: Urine concentration estimate.

• Blood: Detects heme (hemoglobin, myoglobin, intact RBCs).

• Protein: Detects primarily albumin (protein error of indicators).

• Glucose: Glucose oxidase reaction.

• Ketones: Detects acetoacetate

• Bilirubin: Detects conjugated bilirubin.

• Urobilinogen: Intestinal bilirubin metabolite.

• Nitrite: Bacterial nitrate reduction.

• Leukocyte esterase: Neutrophil enzyme.

 

False Positives & False Negatives

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Blood

• False Positives:

  • Myoglobin. Myoglobin has a very short half life and may not be there when checking the urine, but one can rely on + heme with nor RBC’s on microscopy in the right context.

  • Free hemoglobin, i.e. hemoglobinuria from intravascular hemolysis, not urinary tract bleeding.

  • Oxidizing agents

  • Bacterial peroxidase (e.g. lactobacillus, MAY be commensal bacteria, enterobacter, staph, strep)

• False Negatives:

  • Vitamin C

  • High specific gravity

  • RBC lysis in dilute urine

  • See below for more!

 

Protein

• False Positives:

  • Alkaline urine

  • Highly concentrated urine

  • Semen/vaginal contamination

• False Negatives:

  • Dilute urine (see “specific gravity” for more details.)

  • Non-albumin proteins (light chains, tubular proteins)

  • Protein is reported as 1+, 2+ etc, that supposedly can be quantified (see chart below), but oftentimes inaccurate.

  • See below for more !

 

Glucose

• False Positives:

  • Oxidizing contaminants

• False Negatives:

  • Vitamin C

  • Bacterial consumption

  • Improper storage

 

Ketones: Seen in fasting, vomiting, exercise as well  as DKA. Ketones are freely filtered and is present in low glucose states.

• False Positives:

  • Pigmented medications

• False Negatives:

  • β-hydroxybutyrate predominance. (not measured on UA), therefore early DKA may be underestimated. 

  • Delayed analysis of UA.

 

Bilirubin: detected as a result of liver disease/cholestasis. Occasionally due to fasting. Urine is dark.

• False Positives:

  • Pigmented drugs

• False Negatives:

  • Light exposure

  • Oxidation

  • Vitamin C

 

Urobilinogen: detected as a result of RBC breakdown (Hb->biliverdin->bilirubin->conjugated bilirubin (liver)-> urobilinogen (duodenum)->30-50% absorbed in colon back to liver into bile, some into circulation->kidney. Urobilin gives urine its yellow  color. May be detected in hepatocellular disease, but NOT in obstruction

• False Positives:

  • Drug cross-reactivity

• False Negatives:

  • Standing urine oxidation

  • Antibiotics

  • Biliary obstruction

 

Nitrites. SPECIFIC  for UTI but not sensitive, as may be falsely negative, see below.

• False Positives:

  • Standing urine

  • Contamination

• False Negatives:

  • Frequent voiding (needs to be present ~ 4+ hours for bacteria to convert nitrate-> nitrite

  • Non–nitrate reducers (e.g. Enterococcus, Pseudomonas, Staph)

  • Vitamin C

 

Leukocyte Esterase. SENSITIVE for UTI, but not specific since it is elevated in systemic infection/inflammation and contamination. (e.g. sterile pyuria in Kawasaki disease). Cells may also lyse in alkaline urine, yielding neg microscopy

• False Positives:

  • Vaginal contamination

  • Oxidizing contaminants

• False Negatives:

  • High glucose/protein

  • High specific gravity

  • Vitamin C

 

Specific Gravity

• False Positives:

  • Radiocontrast dye

  • High glucose/protein

• False Negatives:

  • Alkaline urine

  • Temperature variation

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Microscopy

• RBCs

  • Glomerular injury (dysmorphic) , lower tract bleeding or trauma (isomorphic). RBC casts indicate GN. 

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• WBCs
  • Neutrophils in sediment via migration. WBC indicate infection or inflammation, casts indicate intrarenal location (AIN, Pyelo)

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• Tubular epithelial cells- tubular injury- ATN/ischemia/Nephrotoxins

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• Casts= Tamm-Horsfall proteins  (uromodulin) produced by tubular in the thick AL, plus one of the contents below
  • Hyaline: non specific- dehydration, fever, exercise, normal.
  • RBC: GN, vasculitis
  • WBC: AIN, pyelo
  • Granular (muddy brown): ATN
  • Epithelial: tubular damage
  • Waxy: CKD
  • Fatty: nephrotic syndrome

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• Crystals: Supersaturation + pH dependent precipitation. like a precursor for a stone
  •  Can U Order a Stone Panel is a mnemonic for stones and pH environment conducive to their formation listed from acidic->basic with oxalate at neutral pH (7) :
  • Cystine, Uric acid, Ca Oxalate, Struvite, Phosphate

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