Lead
Sometime in April or May, a letter arrives from school asking parents to review the results of a urine test. The words "protein +" and "blood +" appear on the form. The child seems perfectly fine. There are no symptoms. Yet from the moment the envelope is opened, the question "is something wrong with her kidneys?" takes hold.
The school urine screening program operates very differently from cancer screening, and most parents find the results hard to interpret intuitively. What does a "+" actually mean? Does "further testing recommended" mean see a doctor today? And when told to "observe," what exactly are you supposed to be watching?
This article explains the three-stage flow of Japan's school urine screening program — a universal annual test that is relatively uncommon outside Japan — and clarifies the difference between findings that are benign and require no treatment and those that warrant formal evaluation.
Background
Japan's school urine screening program is mandated under the School Health and Safety Act (Article 13) and applies to all first-year students in elementary school, junior high school, and high school, every year. The system uses a three-stage funnel: first-stage screening (all students) → second-stage screening (those who tested positive at stage one) → detailed examination (those positive at stage two) [1]. The goal is to detect kidney disease before symptoms appear.
The positive rate at stage one (either protein or blood positive) is approximately 5–8% [1]. Of those, the proportion referred for detailed examination after stage two is roughly 0.3–1%, and the proportion ultimately diagnosed with a kidney disease requiring treatment is smaller still [1].
Understanding this structure is the essential first step. A positive result at stage one means "proceed to the next step" — not "kidney disease is present."
Orthostatic Proteinuria — the Benign Explanation for Most Positives
When protein is flagged in a school urine test, approximately 60–70% of cases turn out to be orthostatic proteinuria: a benign condition where protein appears in the urine while standing or active but disappears in samples collected before getting out of bed; no kidney damage occurs (also called postural proteinuria) — a benign condition in which urinary protein increases when the child is upright and active, but disappears when they are lying down [1]. The mechanism is not fully understood but likely involves increased renal venous pressure when standing, with no structural damage to the glomeruli [6].
The confirmation method is straightforward. Collect a urine sample first thing in the morning — at least two hours after the child last got up — and test it with a dipstick. If the early-morning specimen is protein-negative, orthostatic proteinuria becomes a likely explanation. A 20-year follow-up study by Springberg and colleagues confirmed that orthostatic proteinuria does not impair kidney function over the long term [6].
However, a self-test is not a self-diagnosis. A negative early-morning specimen is information to share with your pediatrician — not a reason to ignore a recommendation for further evaluation.
Asymptomatic Hematuria — Not Missing IgA Nephropathy
When red blood cells are detected in the urine, the finding is called hematuria. It may be macroscopic (visible to the naked eye) or microscopic (detected only on dipstick or microscopy). School urine screening predominantly detects the latter, with no accompanying symptoms.
The main diagnoses to consider in asymptomatic microscopic hematuria include IgA nephropathy: a kidney disease in which IgA antibodies build up in the glomeruli, the kidney's filtering units, sometimes leading to long-term kidney damage, thin basement membrane disease, hypercalciuria, and benign familial hematuria, among others [3]. Of these, IgA nephropathy carries the greatest clinical weight: it is the most common form of chronic glomerulonephritis in Japan [3].
IgA nephropathy is frequently discovered through asymptomatic hematuria or proteinuria on screening. In a subset of patients it can progress to end-stage renal disease. Treatment — tonsillectomy, steroid therapy, renin-angiotensin system inhibitors — is guided by kidney biopsy findings. The KDIGO 2012 chronic kidney disease guideline specifically discusses the balance between the harms of over-investigating isolated microscopic hematuria and the risk of missing a progressive condition [3].
Nephrotic Syndrome — Heavy Proteinuria and Edema
Minimal-change nephrotic syndrome: the most common cause of nephrotic syndrome in children, where the kidneys leak large amounts of protein into the urine; it usually responds well to corticosteroid treatment is among the more common glomerular diseases of school age. The classic presentation is edema of the face, eyelids, and lower limbs together with heavy proteinuria (≥3.5 g/day), low serum albumin, and high cholesterol [5].
On dipstick testing, proteinuria is markedly positive — and, importantly, does not disappear in the early-morning specimen. Because edema is usually visible, this condition is more often brought to attention by symptoms than by screening alone. The response rate to corticosteroid therapy is approximately 80–90%, and with treatment most children achieve remission [5].
IgA Vasculitis Nephritis — Connecting Purpura and the Kidneys
When palpable purpura and joint pain in the abdomen and lower limbs are followed by hematuria and proteinuria, IgA vasculitis (Henoch-Schönlein purpura) with renal involvement must be considered [4]. A review by Davin and Coppo estimated that renal nephritis complicates IgA vasculitis in approximately 20–50% of cases; most are mild, but a subset can lead to persistent renal impairment [4].
"Reddish urine after a rash on the legs" or "an abnormal school urine test in a child who recently had purpura" should prompt evaluation for nephritis.
Putting It into Practice
If a "protein +" notification is received, avoid self-diagnosis. What a family can do is prepare so that the follow-up screening uses an accurate early-morning specimen. On the night before, have the child empty their bladder before sleep; in the morning, collect the sample before the child stands up or starts the day. That distinction — sample taken before standing — is what makes orthostatic proteinuria distinguishable from genuine kidney disease. If the day before the initial screening included intense exercise, a sports event, or club activity, mention that to the clinician as well; it is a meaningful clue for transient proteinuria.
If a detailed examination has been recommended, aim to attend within three months rather than waiting indefinitely. "She seems fine" is not sufficient grounds to postpone. Bring both the stage-one and stage-two result slips to the appointment.
If told "observation only," keep the test result and file it for the following year's screening. A clinician assessing for disease progression — or reassurance — relies on longitudinal data. The annual school urine test is most useful when results are tracked year to year.
Summary
A "+" on the school urine test is the beginning of a confirmation process, not a diagnosis of disease. Most stage-one positives reflect orthostatic proteinuria or other non-progressive findings; only a small fraction require detailed investigation. Yet within that fraction are conditions where early intervention can preserve kidney function. "She seems fine, we'll try again next year" is the one response to avoid. Keeping the test results filed each year and being able to show the longitudinal pattern — that is the minimum habit that turns the school urine screening program from a source of anxiety into a useful medical resource.
References
- Japanese Society for Pediatric Nephrology. Manual for School Urine Screening, 3rd revised edition. 2022. https://www.jspn.jp.
- Japanese Society for Pediatric Nephrology. Clinical Practice Guideline for Pediatric Idiopathic Nephrotic Syndrome 2020. Shindan to Chiryo-sha; 2020.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3(1):1-150. doi:10.1038/kisup.2012.73.
- Davin JC, Coppo R. Henoch-Schönlein purpura nephritis in children. Nat Rev Nephrol. 2014;10(10):563-573. doi:10.1038/nrneph.2014.126. PMID: 25113840.
- Eddy AA, Symons JM. Nephrotic syndrome in childhood. Lancet. 2003;362(9384):629-639. doi:10.1016/S0140-6736(03)14184-0. PMID: 12944064.
- Springberg PD, Garrett LE Jr, Thompson AL Jr, Collins NF, Lordon RE, Robinson RR. Fixed and reproducible orthostatic proteinuria: results of a 20-year follow-up study. Ann Intern Med. 1982;97(4):516-519. doi:10.7326/0003-4819-97-4-516. PMID: 7114631.
- Yoshikawa N, Ito H, Yoshiya K, et al. IgA nephropathy in children from Japan. Child Nephrol Urol. 1989;9(3):191-199. PMID: 2528990.
- Schäfer F, Kirschstein M, Häffner K, et al. Epidemiology of nephrotic syndrome in childhood — lessons from the ESCAPE study. Pediatr Nephrol. 2001;16(12):1037-1040. PMID: 11793094.